positivify := proc(f,vars)
  description "takes a multivariable polynomial and changes its coefficient to positive"
  local c:
  c:=coeffs(f,vars,'t'):
  convert([seq(abs(c[i])*t[i],i=1..nops([t]))],`+`);
end proc:

> LIB "general.lib";  // to have the proc absValue at your disposal
> proc positify(poly f) {
  poly h;
  for (int i=0;i<=size(f);i++) {
  h = h + absValue(leadcoef(f[i]))*leadmonom(f[i]);
}
return(h);
}
> ring r3 = 0,(x,y,z),dp;
> poly F = 4*x^2-x+2xy-3*z^4;
3z4+4x2+2xy+x
> short =0;
_;
> 3*z^4+4*x^2+2*x*y+x

> -2x3y^2;
-4x6y2
> -2*x^3*y^2;
-2x3y2