returns a map representing the substitution i.e. a set of variable/term pairs or null which is used to indicate a failure to unify . concode_field_sep SubstVisitor _substVisitor concode_field_sep List<? extends FOLNode> args concode_elem_sep String op concode_elem_sep Map<Variable,Term> cascadeSubstitution concode_elem_sep Map<Variable,Term> unifyOps concode_elem_sep boolean occurCheck concode_elem_sep boolean isCompound concode_elem_sep Map<Variable,Term> unifyVar