Miniature REPL using OCaml

Implementation of a miniature REPL (e.g. OCaml and Python REPLs). A sample interaction session could be as follows:

>>> 1
1
>>> 2 + 3
5
>>> a = 10
>>> b = 20
>>> c = 2 * a + b
>>> c
40
The above, as you can see allows you to define variables (e.g. a, b and c), and compute complex arithmetic expressions (e.g. 2 * a + b).