Expressions are built from values,
                           constants,
                           character-constants,
                           variables,
                           operators and
                           brackets.

An example of a valid expression is (-3*(q3-q4)+%5)&77., provided that
the registers 3, 4 and 5 do not contain text.

Messages specific to Expressions are:

    Division by 0                                             
    Error in numeric expression                               
    Incorrect character-constant                              
    Illegal octal digit