local env = environment()
local N   = Const("N")
env = add_decl(env, mk_var_decl("N", Type))
env = add_decl(env, mk_var_decl("f", mk_arrow(N, N)))
env = add_decl(env, mk_var_decl("a", N))
local f = Const("f")
local a = Const("a")
local m1   = mk_metavar("m1", mk_metavar("m2", mk_sort(mk_meta_univ("l"))))
local cs   = {}
local ngen = name_generator("tst")
local tc   = type_checker(env, ngen, function (c) print(c); cs[#cs+1] = c end)
assert(tc:num_scopes() == 0)
tc:push()
assert(tc:num_scopes() == 1)
print(tc:check(f(m1)))
assert(#cs == 1)
print(tc:check(f(f(m1))))
assert(#cs == 1) -- New constraint is not generated
tc:pop() -- forget that we checked f(m1)
print(tc:check(f(m1)))
assert(#cs == 2) -- constraint is generated again
check_error(function() tc:pop() end)