code: private UnaryExpression resolveUnary(FunctionRef fun) {
          UnaryExpression operator = null;
          try {
              Class clazz = Class.forName("org.hl7.elm.r1." + fun.getName());
              if (UnaryExpression.class.isAssignableFrom(clazz)) {
                  operator = (UnaryExpression) clazz.newInstance();
                  checkNumberOfOperands(fun, 1);
                  operator.setOperand(fun.getOperand().get(0));
                  builder.resolveUnaryCall("System", fun.getName(), operator);
                  return operator;
              }
          } catch (Exception e) {
              // Do nothing but fall through
          }
          return null;
      }
docs: General Function Support