Function: _<<=_
Class: symbolic_operators
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)

