When no file argument is given, read input from stdin

[vlcinsky]

Many commands are behaving this way.

It has practical advantage e.g. in vim editor, where one can issue:

:%!ramlparser

what would check content of current buffer for correctness even, if it is not saved yet. Content of the buffer gets replaced by command output, but it is easy to read it and then u to undo a step back and correct possible mistakes.