diff --git a/doc/server.org b/doc/server.org
index 6916ebe6b..4e97d5dda 100644
--- a/doc/server.org
+++ b/doc/server.org
@@ -13,13 +13,27 @@ to the standard input.
 ** Load file
 
 #+BEGIN_SRC
-FILE [file-name]
+LOAD [file-name]
 #+END_SRC
 
 This command loads the Lean file named =[file-name]=.
 Lean will create a "snapshot" (aka backtracking point) after each
 command. Lean uses the "snapshots" to process incremental updates efficiently.
 
+** Visit file
+
+#+BEGIN_SRC
+VISIT [file-name]
+#+END_SRC
+
+Lean can keep information about multiple files. This command
+sets =[file-name]= as the "current" file. The remaining  commands
+are all with respect to the current file. If =[file-name]= has not been
+loaded yet, then this command will load it. Some of the remaining commands
+apply "changes" to the current file. The =LOAD= command can be used to
+discard all these changes, and enforce the content of the file stored
+in file system.
+
 ** Replace line
 
 #+BEGIN_SRC
@@ -27,7 +41,7 @@ REPLACE [line-number]
 [new-line]
 #+END_SRC
 
-This command replaces the line =[line-number]= with =[new-line]=.
+This command replaces the line =[line-number]= (in the current file) with =[new-line]=.
 Lean uses the snapshots to process the request efficiently.
 If =[line-number]= is greater than the total number of lines in the lean
 buffer, then empty lines are introduced. The lines are indexed from 1.
@@ -39,7 +53,7 @@ INSERT [line-number]
 [new-line]
 #+END_SRC
 
-This command inserts =[new-line]= before line =[line-number]=.
+This command inserts =[new-line]= (in the current file) before line =[line-number]=.
 If =[line-number]= is greater than the total number of lines in the lean
 buffer, then empty lines are introduced. The lines are indexed from 1.
 
@@ -49,7 +63,7 @@ buffer, then empty lines are introduced. The lines are indexed from 1.
 REMOVE [line-number]
 #+END_SRC
 
-Remove line =[line-number]=. The lines are indexed from 1.
+Remove line =[line-number]= (in the current file). The lines are indexed from 1.
 If =[line-number]= is greater than the total number of lines in the lean
 buffer, then the command is ignored.
 
@@ -59,7 +73,8 @@ buffer, then the command is ignored.
 INFO [line-number]
 #+END_SRC
 
-This command extracts typing information associated with line =[line-number]=.
+This command extracts typing information associated with line
+=[line-number]= (in the current file).
 Lean produces a possible empty sequence of entries terminated with
 
 #+BEGIN_SRC
@@ -108,3 +123,22 @@ num
 -- ACK
 -- ENDINFO
 #+END_SRC
+
+** Check line
+
+As described above, several commands can be used to apply
+modifications to opened/visited files. These modification reflect
+modifications performed by the text editor. The command =CHECK= can be
+used to double check whether the text editor and Lean have the "same
+view" of the current file + modifications.
+
+The following commands returns =-- OK= if the line =[line-number]= in
+the current file is =[line]=. It returns =-- MISMATCH line out of
+range=, if =[line-number]= is too big, and =-- MISMATCH expected
+[lean-line]= when there is a mismatch, and Lean expects
+=[line-number]= to be =[lean-line]=.
+
+#+BEGIN_SRC
+-- CHECK [line-number]
+[line]
+#+END_SRC
diff --git a/src/frontends/lean/server.cpp b/src/frontends/lean/server.cpp
index 42c357920..0d68adf94 100644
--- a/src/frontends/lean/server.cpp
+++ b/src/frontends/lean/server.cpp
@@ -11,15 +11,77 @@ Author: Leonardo de Moura
 #include "frontends/lean/parser.h"
 
 namespace lean {
-server::server(environment const & env, io_state const & ios, unsigned num_threads):
-    m_env(env), m_options(ios.get_options()), m_ios(ios), m_num_threads(num_threads),
-    m_empty_snapshot(m_env, m_options), m_from(0) {
+server::file::file(std::string const & fname):m_fname(fname), m_from(0) {}
+
+void server::file::replace_line(unsigned linenum, std::string const & new_line) {
+    while (linenum >= m_lines.size())
+        m_lines.push_back("");
+    m_lines[linenum] = new_line;
+    if (linenum < m_from)
+        m_from = linenum;
 }
 
-static std::string g_file("FILE");
+void server::file::insert_line(unsigned linenum, std::string const & new_line) {
+    while (linenum >= m_lines.size())
+        m_lines.push_back("");
+    m_lines.push_back("");
+    lean_assert(m_lines.size() >= linenum+1);
+    unsigned i = m_lines.size();
+    while (i > linenum) {
+        --i;
+        m_lines[i] = m_lines[i-1];
+    }
+    m_lines[linenum] = new_line;
+    if (linenum < m_from)
+        m_from = linenum;
+}
+
+void server::file::remove_line(unsigned linenum) {
+    if (linenum >= m_lines.size())
+        return;
+    lean_assert(!m_lines.empty());
+    for (unsigned i = linenum; i < m_lines.size()-1; i++)
+        m_lines[i] = m_lines[i+1];
+    m_lines.pop_back();
+    if (linenum < m_from)
+        m_from = linenum;
+}
+
+/**
+   \brief Return index i <= m_snapshots.size() s.t.
+      * forall j < i, m_snapshots[j].m_line < line
+      * forall i <= j < m_snapshots.size(),  m_snapshots[j].m_line >= line
+*/
+unsigned server::file::find(unsigned linenum) {
+    unsigned low  = 0;
+    unsigned high = m_snapshots.size();
+    while (true) {
+        lean_assert(low <= high);
+        if (low == high)
+            return low;
+        unsigned mid = low + ((high - low)/2);
+        lean_assert(low <= mid && mid < high);
+        lean_assert(mid < m_snapshots.size());
+        snapshot const & s = m_snapshots[mid];
+        if (s.m_line < linenum) {
+            low  = mid+1;
+        } else {
+            high = mid;
+        }
+    }
+}
+
+server::server(environment const & env, io_state const & ios, unsigned num_threads):
+    m_env(env), m_options(ios.get_options()), m_ios(ios), m_out(ios.get_regular_channel().get_stream()),
+    m_num_threads(num_threads), m_empty_snapshot(m_env, m_options) {
+}
+
+static std::string g_load("LOAD");
+static std::string g_visit("VISIT");
 static std::string g_replace("REPLACE");
 static std::string g_insert("INSERT");
 static std::string g_remove("REMOVE");
+static std::string g_check("CHECK");
 static std::string g_info("INFO");
 
 static bool is_command(std::string const & cmd, std::string const & line) {
@@ -40,112 +102,61 @@ static std::string & trim(std::string & s) {
     return ltrim(rtrim(s));
 }
 
-/**
-   \brief Return index i <= m_snapshots.size() s.t.
-      * forall j < i, m_snapshots[j].m_line < line
-      * forall i <= j < m_snapshots.size(),  m_snapshots[j].m_line >= line
-*/
-unsigned server::find(unsigned linenum) {
-    unsigned low  = 0;
-    unsigned high = m_snapshots.size();
-    while (true) {
-        lean_assert(low <= high);
-        if (low == high)
-            return low;
-        unsigned mid = low + ((high - low)/2);
-        lean_assert(low <= mid && mid < high);
-        lean_assert(mid < m_snapshots.size());
-        snapshot const & s = m_snapshots[mid];
-        if (s.m_line < linenum) {
-            low  = mid+1;
-        } else {
-            high = mid;
-        }
-    }
-}
-
 void server::process_from(unsigned linenum) {
-    unsigned i = find(linenum);
-    m_snapshots.resize(i);
-    snapshot & s = i == 0 ? m_empty_snapshot : m_snapshots[i-1];
+    unsigned i = m_file->find(linenum);
+    m_file->m_snapshots.resize(i);
+    snapshot & s = i == 0 ? m_empty_snapshot : m_file->m_snapshots[i-1];
     std::string block;
     lean_assert(s.m_line > 0);
-    m_info.invalidate(s.m_line-1);
-    for (unsigned j = s.m_line-1; j < m_lines.size(); j++) {
-        block += m_lines[j];
+    m_file->m_info.invalidate(s.m_line-1);
+    for (unsigned j = s.m_line-1; j < m_file->m_lines.size(); j++) {
+        block += m_file->m_lines[j];
         block += '\n';
     }
     std::istringstream strm(block);
     m_ios.set_options(s.m_options);
-    parser p(s.m_env, m_ios, strm, m_fname.c_str(), false, 1, s.m_lds, s.m_eds, s.m_line,
-             &m_snapshots, &m_info);
+    parser p(s.m_env, m_ios, strm, m_file->m_fname.c_str(), false, 1, s.m_lds, s.m_eds, s.m_line,
+             &m_file->m_snapshots, &m_file->m_info);
     p.set_cache(&m_cache);
     p();
 }
 
 void server::update() {
-    if (m_from == m_lines.size())
+    if (m_file->m_from == m_file->m_lines.size())
         return;
-    process_from(m_from);
-    m_from = m_lines.size();
+    process_from(m_file->m_from);
+    m_file->m_from = m_file->m_lines.size();
 }
 
-void server::read_file(std::string const & fname) {
+void server::load_file(std::string const & fname) {
     std::ifstream in(fname);
     if (in.bad() || in.fail()) {
-        std::cout << "Error: failed to open file '" << fname << "'";
+        m_out << "-- ERROR failed to open file '" << fname << "'" << std::endl;
     } else {
-        m_fname = fname;
-        m_lines.clear();
+        m_file.reset(new file(fname));
+        m_file_map.insert(mk_pair(fname, m_file));
         for (std::string line; std::getline(in, line);) {
-            m_lines.push_back(line);
+            m_file->m_lines.push_back(line);
         }
-        m_from = 0;
     }
 }
 
-void server::replace_line(unsigned linenum, std::string const & new_line) {
-    while (linenum >= m_lines.size())
-        m_lines.push_back("");
-    m_lines[linenum] = new_line;
-    if (linenum < m_from)
-        m_from = linenum;
-}
-
-void server::insert_line(unsigned linenum, std::string const & new_line) {
-    while (linenum >= m_lines.size())
-        m_lines.push_back("");
-    m_lines.push_back("");
-    lean_assert(m_lines.size() >= linenum+1);
-    unsigned i = m_lines.size();
-    while (i > linenum) {
-        --i;
-        m_lines[i] = m_lines[i-1];
-    }
-    m_lines[linenum] = new_line;
-    if (linenum < m_from)
-        m_from = linenum;
-}
-
-void server::remove_line(unsigned linenum) {
-    if (linenum >= m_lines.size())
-        return;
-    lean_assert(!m_lines.empty());
-    for (unsigned i = linenum; i < m_lines.size()-1; i++)
-        m_lines[i] = m_lines[i+1];
-    m_lines.pop_back();
-    if (linenum < m_from)
-        m_from = linenum;
+void server::visit_file(std::string const & fname) {
+    auto it = m_file_map.find(fname);
+    if (it == m_file_map.end())
+        load_file(fname);
+    else
+        m_file = it->second;
 }
 
 void server::show_info(unsigned linenum) {
     update();
-    unsigned i = find(linenum);
-    environment const & env = i == 0 ? m_env     : m_snapshots[i-1].m_env;
-    options const & o       = i == 0 ? m_options : m_snapshots[i-1].m_options;
+    unsigned i = m_file->find(linenum);
+    environment const & env = i == 0 ? m_env     : m_file->m_snapshots[i-1].m_env;
+    options const & o       = i == 0 ? m_options : m_file->m_snapshots[i-1].m_options;
     m_ios.set_options(o);
     io_state_stream out(env, m_ios);
-    m_info.display(out, linenum);
+    m_file->m_info.display(out, linenum);
     out << "-- ENDINFO" << endl;
 }
 
@@ -164,23 +175,62 @@ unsigned server::get_linenum(std::string const & line, std::string const & cmd)
     return r;
 }
 
+void server::check_file() {
+    if (!m_file)
+        throw exception("no file has been loaded/visited");
+}
+
+void server::replace_line(unsigned linenum, std::string const & new_line) {
+    check_file();
+    m_file->replace_line(linenum, new_line);
+}
+
+void server::insert_line(unsigned linenum, std::string const & new_line) {
+    check_file();
+    m_file->insert_line(linenum, new_line);
+}
+
+void server::remove_line(unsigned linenum) {
+    check_file();
+    m_file->remove_line(linenum);
+}
+
+void server::check_line(unsigned linenum, std::string const & line) {
+    check_file();
+    if (linenum >= m_file->m_lines.size()) {
+        m_out << "-- MISMATCH line out of range" << std::endl;
+    } else if (m_file->m_lines[linenum] != line) {
+        m_out << "-- MISMATCH expected " << m_file->m_lines[linenum] << std::endl;
+    } else {
+        m_out << "-- OK" << std::endl;
+    }
+}
+
 bool server::operator()(std::istream & in) {
     for (std::string line; std::getline(in, line);) {
         try {
-            if (is_command(g_file, line)) {
-                std::string fname = line.substr(g_file.size());
+            if (is_command(g_load, line)) {
+                std::string fname = line.substr(g_load.size());
                 trim(fname);
-                read_file(fname);
+                load_file(fname);
+            } else if (is_command(g_visit, line)) {
+                std::string fname = line.substr(g_visit.size());
+                trim(fname);
+                visit_file(fname);
+            } else if (is_command(g_check, line)) {
+                unsigned linenum = get_linenum(line, g_check);
+                read_line(in, line);
+                check_line(linenum-1, line);
             } else if (is_command(g_replace, line)) {
                 unsigned linenum = get_linenum(line, g_replace);
                 read_line(in, line);
                 replace_line(linenum-1, line);
             } else if (is_command(g_insert, line)) {
-                unsigned linenum = get_linenum(line, g_replace);
+                unsigned linenum = get_linenum(line, g_insert);
                 read_line(in, line);
                 insert_line(linenum-1, line);
             } else if (is_command(g_remove, line)) {
-                unsigned linenum = get_linenum(line, g_replace);
+                unsigned linenum = get_linenum(line, g_remove);
                 remove_line(linenum-1);
             } else if (is_command(g_info, line)) {
                 unsigned linenum = get_linenum(line, g_info);
@@ -189,7 +239,7 @@ bool server::operator()(std::istream & in) {
                 throw exception(sstream() << "unexpected command line: " << line);
             }
         } catch (exception & ex) {
-            std::cout << "Error: " << ex.what() << "\n";
+            m_out << "-- ERROR " << ex.what() << std::endl;
         }
     }
     return true;
diff --git a/src/frontends/lean/server.h b/src/frontends/lean/server.h
index 7f8a6d0c7..e3343c36c 100644
--- a/src/frontends/lean/server.h
+++ b/src/frontends/lean/server.h
@@ -7,6 +7,7 @@ Author: Leonardo de Moura
 #pragma once
 #include <vector>
 #include <string>
+#include <unordered_map>
 #include "library/definitions_cache.h"
 #include "frontends/lean/parser.h"
 #include "frontends/lean/info_manager.h"
@@ -17,22 +18,38 @@ namespace lean {
    external processes.
 */
 class server {
-    std::vector<std::string>  m_lines;
-    snapshot_vector           m_snapshots;
-    info_manager              m_info;
+    struct file {
+        std::string               m_fname;
+        std::vector<std::string>  m_lines;
+        snapshot_vector           m_snapshots;
+        info_manager              m_info;
+        unsigned                  m_from; // mark the first modified line that was not processed yet.
+
+        file(std::string const & fname);
+        unsigned find(unsigned linenum);
+        void replace_line(unsigned linenum, std::string const & new_line);
+        void insert_line(unsigned linenum, std::string const & new_line);
+        void remove_line(unsigned linenum);
+    };
+    typedef std::shared_ptr<file>                     file_ptr;
+    typedef std::unordered_map<std::string, file_ptr> file_map;
+    file_map                  m_file_map;
+    file_ptr                  m_file;
     environment               m_env;
     options                   m_options;
     io_state                  m_ios;
+    std::ostream &            m_out;
     unsigned                  m_num_threads;
     snapshot                  m_empty_snapshot;
-    std::string               m_fname;
-    unsigned                  m_from;
     definitions_cache         m_cache;
 
-    void read_file(std::string const & fname);
+    void load_file(std::string const & fname);
+    void visit_file(std::string const & fname);
+    void check_file();
     void replace_line(unsigned linenum, std::string const & new_line);
     void insert_line(unsigned linenum, std::string const & new_line);
     void remove_line(unsigned linenum);
+    void check_line(unsigned linenum, std::string const & line);
     void show_info(unsigned linenum);
     void process_from(unsigned linenum);
     unsigned find(unsigned linenum);