local m = mk_metavar("m", Prop)
local s = substitution()
assert(not s:is_assigned(m))
assert(not s:is_expr_assigned("m"))
assert(not s:is_level_assigned("m"))
local f = Const("f")
local g = Const("g")
local a = Const("a")
local t = f(f(a))
s = s:assign(m, t)
assert(s:is_assigned(m))
assert(s:is_expr_assigned("m"))
assert(not s:is_level_assigned("m"))
assert(s:instantiate(g(m)) == g(t))
s = s:assign("m", a)
assert(s:instantiate(g(m)) == g(a))
local l = mk_level_one()
local u = mk_meta_univ("u")
s = s:assign(u, l)
assert(s:is_assigned(u))
assert(s:is_level_assigned("u"))
assert(not s:is_expr_assigned("u"))
assert(s:get_expr("m") == a)
local m2 = mk_metavar("m2", Prop)
s = s:assign(m2, f(m))
print(s:get_expr("m2"))
assert(s:occurs(m, f(m2)))
assert(s:occurs_expr("m", f(m2)))
print(s:get_level("u"))
print(s:instantiate(mk_sort(u)))
assert(s:instantiate(mk_sort(u)) == mk_sort(l))
assert(s:get_assignment(m) == a)
assert(s:get_assignment(u) == l)
assert(s:get_expr_assignment("m") == a)
assert(s:get_level_assignment("u") == l)