complex generate usage in multiplier

Hi all,

I am trying to build a multiply by constant multiplier (using
shifts & additions/subtractions). The multiplier is to be a generic
vector 1 to 15.

Basically I should enter "101Z00000000000", this should infer I need
one adder & one subtractor. The 'Z' means -1 in canonical form.

The for loop should parse that vector & for the the first 2 ones
generate an adder, & for the next Z generate a subtractor and so one.
I can manage the port mapping (hopefully), I just hope someone can
guide me with this form of 'generate'.

BTW, can I use variables with generate?? I want to keep track of
whether or not the current bit is one of the first 2 bits, but
couldn't

Any help is highly appreciated.

Ahmad