In Lisp, a function which returns a boolean `true' or boolean `false'
value is called a predicate. As is the convention in Lisp a
value of nil means false, anything else means true. The symbol
t is often used to represent a true value (in fact, sometimes
the symbol t should be read as any non-nil value).
Another Lisp convention is that the names of predicate functions should be the concept the predicate is testing for and either `p' or `-p'.
The `p' variant is used when the concept name does not contain any hyphens.
For example a predicate to test for the concept const-variable (a
variable which has a constant value, see section Constant Variables) would
be called const-variable-p. On the other hand a predicate
to test for the concept buffer (a Lisp object which is a buffer)
would be called bufferp.
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