r     : Identifier | Integer |
           | Identifier "[" Expression "]"
           | "TRUE" | "FALSE" | "(" Expression ")"
           | "~" Factor | "-" Factor.
</pre>

    Each subexpression (<em>SimpleExp</em>, <em>Term</em> and
    <em>Factor</em>) may be evaluated to yield an object of a certain
    type.  The operands of an expression may be evaluated in an
    arbitrary order unless the operator is "OR" or "AND" (see below).<p>

      The following table shows all Obr operators with their the
      