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Again, I'm trying to understand Martin Henson's "Elements of Functional Languages." He talks about "maximal free expression." For example,

M of EXP is a maximal free expression of N of EXP iff M is a free expression of N, and whenever M is a proper subexpression of M', M' is not a free expression of N.

And then comes the exercise:

Identify the free and maximal free expressions of λxy.xzy(zwx)(wwzyw)

I cannot envision what an expression is in this context. So if x + y +2z is an expression, would x + y be a subexpression thereof?

. . . and no, I'm not a student. This is all just independent research for someone who has coded but not truly understood. . . .

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