Atom3 forms the concatenation of Atom1 and Atom2.
At least two of the arguments must be instantiated to atoms. This
predicate also allows for the mode (-,-,+), non-deterministically
splitting the 3rd argument into two parts (as append/3
does for lists). SWI-Prolog allows for atomic arguments. Portable code
must use atomic_concat/3
if non-atom arguments are involved.