Loosely used to mean "in accordance with all the relevant
rules", especially in connection with some set of constraints
defined by software. "The older =+ alternate for += is no
longer legal syntax in ANSI C." "This parser processes each
line of legal input the moment it sees the trailing linefeed."
Hackers often model their work as a sort of game played with
the environment in which the objective is to maneuver through
the thicket of "natural laws" to achieve a desired objective.
Their use of "legal" is flavoured as much by this game-playing
sense as by the more conventional one having to do with courts