The X=..L predicate (pronounced "univ") provides a way to obtain the arguments of a structure as a list or construct a structure or atom from a list of arguments.
Examples
?- p(a,b,c) =.. X.
X = [p,a,b,c]
yes
?- p(a,b,c) =.. [p,a,b,c].
yes
?- p(a,b,c) =.. [p,x,y,z].
no
?- p(a,b,c) =.. [].
no
?- [a,b,c,d] =.. X.
X = [.,a,[b,c,d]]
yes
?- [a,b,c,d] =.. [X|Y].
X = .
Y = [a,[b,c,d]]
yes
?- X =.. [a,b,c,d].
X = a(b, c, d)
yes
?- X =.. [a,[b,c],d].
X = a([b,c], d)
yes
?- a+b =.. X.
X = [+,a,b]
yes
?- a+b =.. [+, X, Y].
X = a
Y = b
yes
?- a =.. [a].
yes
?- a =.. [b].
no
?- p =.. [p,x,y,z].
no
?- p(a,b,c) =.. [p].
no
?- X =.. [a].
X = a
yes
?- a+b =.. '+ X Y'.
Expected second argument to be a variable or a list but got a ATOM with value: + X Y
?- X =.. Y.
Both arguments are variables: X and: Y