_line_ __= _conditional_ $

	<line>        ::= <pipeline>
	<pipeline>    ::= <cmd> (<pipe> | E)
	<pipe>        ::= "|" <pipeline>
	<cmd>         ::= <cmd_elem>+
	<cmd_elem>    ::= <string> | <redirection>
	<redirection> ::= ("<" | ">" | "<<" | ">>") <string>

	$ => end of file
	E => nothing
	+ => at least one occurence
	recursive-descent-parser