code: public StringExpression substring(Expression<Integer> beginIndex, Expression<Integer> endIndex) {
          return Expressions.stringOperation(Ops.SUBSTR_2ARGS, mixin, beginIndex, endIndex);
      }
docs: Create a {@code this.substring(beginIndex, endIndex)} expression
  
  @param beginIndex inclusive start index
  @param endIndex exclusive end index
  @return this.substring(beginIndex, endIndex)
  @see java.lang.String#substring(int, int)