An arithmetic expression can be one of the following:

    1)  An identifier (including function-identifier)
        of a numeric elementary item
    2)  A numeric literal
    3)  Two or more of the above choices separated by arithmetic
        operators (+, -, *, /, **)
    4)  Two or more arithmetic expressions separated by an arithmetic
        operator (+, -, *, /, **)
    5)  An arithmetic expression enclosed in parentheses

  A unary operator (a sign) can precede any arithmetic expression.

  The identifiers and literals in an arithmetic expression must represent
  either: (1) numeric elementary items (including functions),
  or (2) numeric literals on which arithmetic can be performed.