public Frame unify(RBTerm other, Frame f) {
// System.err.println("** entering unify " + this + " to: " + other);
if (other instanceof RBIgnoredVariable) return f;
RBTerm val = f.get(this);
if (val == null) {
other = other.substitute(f);
if (equals(other)) return f;
else if (other.freefor(this)) {
return bind(other, f);
} else return null;
} else return val.unify(other, f);
}
