Output message when reading from standard input about tchecker HOT 2 OPEN

Do you really expect users to type the input model on stdin?

What I do in Spot is that when there is no specified input, I use isatty(STDIN_FILENO) to check if stdin is a terminal. In that case I simply display an error message like this:

% ltl2tgba
ltl2tgba: No formula to translate?  Run 'ltl2tgba --help' for help.
Use 'ltl2tgba -' to force reading formulas from the standard input.

Entering formulas interactively is still possible, as the message points out, but without arguments it's much more likely that some user is just trying to figure out how to run the tool, so I prefer to abort.

If stdin has been redirected from a file or pipe, the isatty() check will not abort, and passing - explicitly is not necessary.

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Thanks for the advice, it is only meant to be used in piped commands

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