code: @Override
      public JavaShellToolBuilder err(PrintStream cmdErr, PrintStream userErr) {
          this.cmdErr = cmdErr;
          this.userErr = userErr;
          return this;
      }
docs: Set the error channels.
  Default, if not set, {@code err(System.err, System.err, System.err)}.
  
  @param cmdErr destination of tool start-up and fatal errors
  @param userErr destination of user code error output.
  For example, user snippet  {@code System.err.println("Oops")}
  when executed {@code Oops} goes to userErr.
  @return the {@code JavaShellToolBuilder} instance