Multiple substitutions (wishlist)

[Jeroen Demeyer <j.demeyer@ugent.be>]

Hello list,

Many times I have wanted to do some kind of "multiple simultaneous 
subst()" in PARI/GP.  I mean the following:
gp> subst(x^2 + 3*y, x,y, y,z)
%1 = y^2 + 3*z

Where the 2nd argument is replaced by the 3rd, the 4th by the 5th, and 
so on...
I have absolutely no idea how easy/difficult it would be to implement 
this, but I would appreciate it :-)
One non-trivial point it that the subst()s have to happen 
simultaneously, to make things like subst(f, x,y, y,x) possible (which 
would swap x and y in the expression f).
Cheers,
Jeroen