code: @NonNull
      public Expression add(@NonNull Expression expression) {
          if (expression == null) {
              throw new IllegalArgumentException("expression cannot be null.");
          }
          return new BinaryExpression(this, expression, BinaryExpression.OpType.Add);
      }
docs: Create an add expression to add the given expression to the current expression
  
  @param expression an expression to add to the current expression.
  @return an add expression.