SWI-Prolog -- explain/1

Availability::- use_module(library(explain)).(can be autoloaded)

[det]explain(@Term)

Give an explanation on Term. The argument may be any Prolog data object. If the argument is an atom, a term of the form Name/Arity or a term of the form Module:Name/Arity, explain/1 describes the predicate as well as possible references to it. See also gxref/0.