element. The nodes could pass a small number of messages called markers along the links
under the control of the central computer. This operation of marker-passing was used to
perform inheritance and categorization.
NETL's marker passing system had major limitations which have been discussed by
Fahlman as well as others [6,7,3,2]. The answers provided by the network were sensi-
tive to race conditions, especially in the presence of exceptions and multiple hierarchies.
There were even more serious problems with the semantics (or the lack of it) of the IS-A
links,
given the existence of CANCEL links
3
. Another limitation of the marker passing
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