I have a set of activities which sometimes include other activities; this is
recursive, the result resembles a tree structure, similar to a directory
tree.  Until now I've quite simply disallowed 'multiple parents', but
largely because my brain's too feeble to see a cleaner loop-avoidance
mechanism.

(Clear as mud?  Here I have A includes B & D, B includes C, E includes F,
all just lovely.  This works nicely even in a physical sense, and looks like
a tree.)

ActA
 |-ActB
 |-ActC
    |-ActD
ActE
 |-ActF

Now I need to add ActF to ActC, basically meaning I want to allow leaf F
appear on multiple trees.  This is ok, but the procedure of adding ActF to
ActC must include enough exhaustive checking to ensure that I disallow
creation of loops, such as that which would occur if I were to add ActA to
ActC.

Can anyone here on good old HP3000-L point me in the direction of the
perfect solution?  (I just KNOW there are papers out there, so even a good
Google key would be a great pointer!)

TIA

Tracy Pierce

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