A pattern matching question

Here's a very simple input file (the mathematics is meaningless; I'm only
interested in syntax here):

tt:=operator 'tt

ttrules := rule
  tt(n^(a | integer?(a))) == t/a
  tt(n/(n-1)) == 1/t
  tt(n/(n-1)^(a | integer?(a) and a>1)) == 1/t^a

And here's some experiments:

(1) -> ttrules tt(n^6)

         t
   (1)   -
         6
                                                     Type: Expression
Integer
(1) -> ttrules tt(n/(n-1))

         1
   (1)   -
         t
                                                     Type: Expression
Integer
(3) -> ttrules tt(n/(n-1)^2)

                 n
   (3)   tt(-----------)
             2
            n  - 2n + 1
                                                     Type: Expression
Integer

Why is the last pattern not matched?

Thanks,
Alasdair

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