1. A factor is an expression that can not be broken apart by any adjacent oper-
ator, either a * or a +. The only factors in our expression language are:
(a) Identifiers. It is not possible to separate the letters of an identifier by
attaching an operator.
(b) Any parenthesized expression, no matter what appears inside the paren-
theses. It is the purpose of parentheses to prevent what is inside from
becoming the operand of any operator outside the parentheses.