By convention, the names of procedures that always return a boolean
value usually end
in “?”. Such procedures are called predicates.
By convention, the names of procedures that store values into previously
allocated locations (see section see Storage model) usually end in
“!”.
Such procedures are called mutation procedures.
By convention, the value returned by a mutation procedure is unspecified.
By convention, “->” appears within the names of procedures that
take an object of one type and return an analogous object of another type.
For example, list->vector takes a list and returns a vector whose
elements are the same as those of the list.