Variables and numbers are members of a syntactic category known as
primary operands, or just primaries.
Informally, a primary is a syntactic construct that
can appear as the operand of any operator without
requiring parenthesization. In fact, when parentheses are used to
change the order of operations, the parenthesized expression is
considered a primary, although it can itself contain arbitrarily
complex subexpressions. Parenthesized expressions are just one example
of complex primary expressions. Others are introduced in the
sections that follow this one and are summarized in §2.10.1.
Figure 2.5 shows the structure of primary operands.
