Function: _<<=_
Section: symbolic_operators
C-Name: gshiftle
Prototype: &L
Help: x<<=y: shortcut for x=x<<y.
Description:
 (*small, small):small:parens             $1 <<= $(2)
 (*int, small):int:parens                 $1 = shifti($1, $2)
 (*mp, small):mp:parens                   $1 = mpshift($1, $2)
 (*gen, small):mp:parens                  $1 = gshift($1, $2)
