 <h3>Input</h3>

We provide a structure [[Read]] that contains a modified form of the
input reader used on the Scheme interpreter.
It satisfies the signature:
<<read-sig.sml>>=
signature READ = sig
  datatype ipt = INT of int
               | SYMBOL of string
               | PARENS of ipt list
  exception EOF
  exception PrematureEOF of string
  val prompt_read : string -> ipt list
  val read : (unit -> string) -> string -> ipt list
end
@ %def prompt_read READ read ipt EOF PrematureEOF
You will want to use [[prompt_read]] to prompt for a move and
implement [[readmove]].  Be sure to handle the exceptions properly.