Conditional Expressions
=======================

   A "conditional expression" is a special kind of expression with
three operands.  It allows you to use one expression's value to select
one of two other expressions.

   The conditional expression looks the same as in the C language:

     SELECTOR ? IF-TRUE-EXP : IF-FALSE-EXP

There are three subexpressions.  The first, SELECTOR, is always
computed first.  If it is "true" (not zero and not null) then
IF-TRUE-EXP is computed next and its value becomes the value of the
whole expression.  Otherwise, IF-FALSE-EXP is computed next and its
value becomes the value of the whole expression.

   For example, this expression produces the absolute value of `x':

     x > 0 ? x : -x

   Each time the conditional expression is computed, exactly one of
IF-TRUE-EXP and IF-FALSE-EXP is computed; the other is ignored.  This
is important when the expressions contain side effects.  For example,
this conditional expression examines element `i' of either array `a' or
array `b', and increments `i'.

     x == y ? a[i++] : b[i++]

This is guaranteed to increment `i' exactly once, because each time one
or the other of the two increment expressions is executed, and the
other is not.