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