  The starting point of this work are inaccurate statements found in the
literature for Multi-terminal Binary Decision Diagrams (MTBDDs) regarding the
well-definedness of the MTBDD abstraction operation. The statements try to
relate an operation * on a set of terminal values M to the property that the
abstraction over this operation does depend on the order of the abstracted
variables. This paper gives a necessary and sufficient condition for the
independence of the abstraction operation of the order of the abstracted
variables in the case of an underlying monoid and it treats the more general
setting of a magma.
