Legal/Permission Question

[Dan]

Hey Walter/anyone?

Considering that my Walnut 2.x engine performs much the same functionality as DMDScript, but that Walnut 2.x is written significantly differently on a structural level...

DMDScript is a GPL v1 engine, and Walnut 2.x is a new BSD engine.

When I fill in the function stubs for, for example, Number_prototype_toFixed, am I allowed to examine Walter's DMDScript source code, write something similar (but obviously not the same) and still call it new BSD?

What requirements are there to do such a thing?