void push(Register s,Operand name){
  Stack<Operand> stack=get(s);
  if (stack == null) {
    stack=new Stack<Operand>();
    put(s,stack);
  }
  stack.push(name);
}

void push ( Register s , Operand name ) { Stack < Operand > stack = get ( s ) ; if ( stack == null ) { stack = new Stack < Operand > ( ) ; put ( s , stack ) ; } stack . push ( name ) ; }
push a name at the top of the stack for a particular register
