code: public static ExprString toExprString(Expression expr) {
  	if (expr instanceof ExprString) return (ExprString) expr;
  	if (expr instanceof Literal) return expr.getFactory().createLitString(((Literal) expr).getString(), expr.getStart(), expr.getEnd());
  	return new CastString(expr);
      }
docs: Create a String expression from a Expression
  
  @param expr
  @param pos
  @return String expression