code: public UpdateForClause within(String variable, String path) {
          vars.add(x(variable + " WITHIN " + path));
          return this;
      }
docs: Adds a "<code>variable WITHIN path</code>" section to the clause.
  
  @param variable the next variable to add to the clause.
  @param path the path for the variable, a WITHIN path.
  @return the clause, for chaining. See {@link #when(Expression)} and {@link #end()} to complete the clause.