Function: shiftmul
Section: operators
C-Name: gmul2n
Prototype: GL
Help: shiftmul(x,n): multiply x by 2^n (n>=0 or n<0).
Doc: multiplies $x$ by $2^n$. The difference with
 \kbd{shift} is that when $n<0$, ordinary division takes place, hence for
 example if $x$ is an integer the result may be a fraction, while for shifts
 Euclidean division takes place when $n<0$ hence if $x$ is an integer the result
 is still an integer.
