Symbolic simplification for pretty output

This is a crosspost from http://stackoverflow.com/questions/21512047/matlab-mupad-symbolic-simplification-for-pretty-output

I need to automatically simplify some symbolic expressions but the `simplify` function of matlab can't do everything i need.

Example:

    simplify(expand((ax + bx)^2 + (ay + by)^2))

Which results in the output

    ax^2 + 2*ax*bx + ay^2 + 2*ay*by + bx^2 + by^2

So i've tried to create my own rules to make the output prettier

    function [ result ] = simplify_pretty( term )
        read(symengine, 'simplify_pretty_rules.mu');
        result = feval(symengine, 'Simplify', term, 'SelectRules = simplify_pretty_rules')
    end

with "simplify_pretty_rules.mu"

    simplify_pretty_rules := proc()
    begin
        [
            Rule(#X^2 - 2 * #X * #Y + #Y^2, (#X - #Y)^2),
            Rule(#X^2 + 2 * #X * #Y + #Y^2, (#X + #Y)^2)
        ]
    end_proc:

This works for input like...

    simplify_pretty(expand((ax + bx)^2 + (ay + by)^2))

...but when changed to...

    simplify_pretty(expand(-(ax + bx)^2 - (ay + by)^2))

...it doesn't

So the questions is: Is it possible to create rules which work in (nearly) all situations? What am I doing wrong?