if Special FormAnother special form is the conditional if. This form is used
to instruct the computer to make decisions. You can write function
definitions without using if, but it is used often enough, and
is important enough, to be included here. It is used, for example, in
the code for the function beginning-of-buffer.
The basic idea behind an if, is that if a test is true,
then an expression is evaluated. If the test is not true, the
expression is not evaluated. For example, you might make a decision
such as, “if it is warm and sunny, then go to the beach!”