code: public NumberExpression<Integer> indexOf(Expression<String> str) {
          return Expressions.numberOperation(Integer.class, Ops.INDEX_OF, mixin, str);
      }
docs: Create a {@code this.indexOf(str)} expression
  
  <p>Get the index of the given substring in this String</p>
  
  @param str string
  @return this.indexOf(str)
  @see java.lang.String#indexOf(String)