### learn rules: !=>

```
!=> :
description:
op |ket> !=> sequence
op |ket> !=>
statements ...

associate the uncompiled sequence with op|ket>
this "sequence" can be any type of BaseSequence
on invocation of op|ket>, the sequence is compiled and then returned
and further, op|ket> is set to that final result
ie, this is a memoizing operator definition
useful when you want to store the results of (expensive) computations so you don't need to recalculate them
toy examples where this is useful are Fibonacci and factorials
frequently used with label-descent

examples:
-- the standard example, Fibonacci:
Fib |0> => |0>
Fib |1> => |1>
Fib |*> !=> Fib minus[1] |_self> ++ Fib minus[2] |_self>

-- see what we know before invocation:
sa: dump
------------------------------------------
|context> => |Global context>

Fib |0> => |0>
Fib |1> => |1>
Fib |*> !=> Fib minus[1]|_self> ++ Fib minus[2]|_self>
------------------------------------------

-- now find Fibonacci 10:
sa: Fib |10>
|55>

-- now see what we know:
sa: dump
------------------------------------------
|context> => |Global context>

Fib |0> => |0>
Fib |1> => |1>
Fib |*> !=> Fib minus[1]|_self> ++ Fib minus[2]|_self>
Fib |2> => |1>
Fib |3> => |2>
Fib |4> => |3>
Fib |5> => |5>
Fib |6> => |8>
Fib |7> => |13>
Fib |8> => |21>
Fib |9> => |34>
Fib |10> => |55>
------------------------------------------