(scientific computation) An extra option added to a routine
without changing the calling sequence. For example, instead
of adding an explicit input variable to instruct a routine to
give extra diagnostic output, the programmer might just add a
test for some otherwise meaningless feature of the existing
inputs, such as a negative mass. The use of hidden flags can
make a program very hard to debug and understand, but is all
too common wherever programs are hacked in a hurry.