PrologTerm.java

    @Override
    public boolean equals(Object o) {
        if (o instanceof PrologTerm) {
            PrologTerm t = (PrologTerm) o;
            return this.equals(t);
        } else {
            System.out.println("Strange");
            return false;
        }
    }

    public boolean equals(PrologTerm o) {
        boolean res = true;
        System.out.println("checking equality of " + this + " and " + o);
        // TODO: throw exception if of another non related type?
        // check if both variables
        if ((o.isVariable() && this.isVariable()))
            return this.equals(o);
        
        // one is a variable and the other is not
        if ((o.isVariable() && !(this.isVariable())) ||
            (this.isVariable() && !(o.isVariable())))
            return false;

        // both back reference, TODO: possible infinite loops here
        if ((this.isBackRef() && (o.isBackRef())))
            ((BackRef) this).equals((BackRef) o);

        // just inverting the order
        if ((this.isBackRef() && !(o.isBackRef())))
            return o.equals(this);

        // only the first one is a back reference
        if ((o.isBackRef() && !(this.isBackRef()))) {
            // equality with the thing pointed
            return ((BackRef) o).getPointed().equals(this);
        }
        
        // in this last case they're both surely not Back Ref, so they have some children
        if (!(this.name.equals(o.name)) || (this.getArity() != o.getArity()))
            return false;
        
        for (int i = 0; i < this.getArity(); i++) {
            if (!(this.getArgument(i).equals(o.getArgument(i)))) {
                return false;
            }
        }
        
        return true;
    }