code: @NonNull
      public static Expression contains(@NonNull Expression expression, @NonNull Expression substring) {
          if (expression == null || substring == null) {
              throw new IllegalArgumentException("expression and substring cannot be null.");
          }
          return new Expression.FunctionExpression("CONTAINS()", Arrays.asList(expression, substring));
      }
docs: Creates a CONTAINS(expr, substr) function that evaluates whether the given string
  expression conatins the given substring expression or not.
  
  @param expression The string expression.
  @param substring  The substring expression.
  @return The CONTAINS(expr, substr) function.