code: private Expression concat( Expression left, char operator, Expression right ) {
          if( left == null ) {
              return right;
          }
          Operation op;
          if( left.getClass() == Operation.class && ((Operation)left).getOperator() == operator ) {
              op = (Operation)left;
          } else if( right != null && right.getClass() == Operation.class && ((Operation)right).getOperator() == operator ) {
              op = (Operation)right;
              op.addLeftOperand( left );
              return op;
          } else {
              op = new Operation( reader, left, operator );
          }
          if( right != null ) {
              op.addOperand( right );
          }
          return op;
      }
docs: Concatenate 2 expressions to one expression.
  
  @param left
  the left, can be null
  @param operator
  the expression operation
  @param right
  the right, can not be null
  @return the resulting expression