Fix multi-operand commutativity #16
Description
Single-operand commutativity works. e.g: 1+2 == 2+1.
Multi-operand commutativity currently is not supported. e.g: 1+2+3 should == 3+1+2, but the system currently doesn't recognise that.
Originally posted by @JoshdRod in #8 (comment)
