code: public StringExpression upper() {
          if (upper == null) {
              upper = Expressions.stringOperation(Ops.UPPER, mixin);
          }
          return upper;
      }
docs: Create a {@code this.toUpperCase()} expression
  
  <p>Get the upper case form</p>
  
  @return this.toUpperCase()
  @see java.lang.String#toUpperCase()