I just read back http://www.paulgraham.com/ilc03.html and found intersting things dealing with what we are talking about (pg doesn't answer, so I'll make hime talk myself ;)
"Letting people rewrite your language is a good idea. You, as the language designer, can't possibly anticipate all the things programmers are going to want to do with it. To the extent they can rewrite the language, you don't have to."
"If you want to overload existing operators to do the right thing when given your new type, you don't need anything new in the core. As long as you have lexical scope, you can just wrap a new definition of the operator around the old one."
" At the conference [pg gave this talk], John McCarthy pointed out that a function to invert a matrix might be better described by writing "inverts a matrix" than by giving the source."
