e functions.  We begin with two basic properties
| of inclusion functions whose proof is obvious and follow this
| with an example that simplifies a compound conditional statement.
|
| 25.  Proposition.  Let A and B be subsets of X.  Then:
|
|      a.  inc_A inc_B  =  inc_(A |^| B)  =  inc_B inc_A.
|
|      b.  If    A |^| B  =  Ø,
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|          then  inc_A + inc_B exists,
|
|          and   inc_A + inc_B  =  inc_(A |_| B).
|
| 26.  Example.  If A, B c X, and f, g, h in SC(X, Y), then:
|
|      if A then (i