the program
    (which is always "integer") or real in which case the real value
    is truncated before being returned.

    <h3>Expressions</h3>

    Expression structure in Obr is determined by operator precedence and
    associativity in the usual way:

<pre>
Expression : SimpleExp | Expression RelOp SimpleExp.
RelOp      : "=" | "#" | "<" | ">".
SimpleExp  : Term | SimpleExp AddOp Term.
AddOp      : "+" | "-" | "OR".
Term       : Factor | Term MulOp Factor.
MulOp      : "*" | "/" | "MOD" | "AND".
Facto