Availability:Arithmetic function (see is/2) 
[ISO]+IntExpr1 << +IntExpr2Bitwise shift IntExpr1 by IntExpr2 bits to the 
left. The ISO standard dictates shifting a negative value is
implementation defined. SWI-Prolog defines shifting negative 
integers to be defined as -(-Int<<Shift). If IntExpr2 
is negative, a left shift (see <</2) 
is performed with the negated value of IntExpr2.