( FFA Ch 4 Puzzle: Write a FFACalc tape that will take seven numbers,
presumed to be on the top of the stack, and return the largest.

For instance, suppose file numbers.txt were to contain:

.9.1.7.5.1.1.0

Then the following example invocation:

cat numbers.txt youranswer.txt |  ./bin/ffa_calc 256 16

... should produce the output:

0000000000000000000000000000000000000000000000000000000000000009 )

( Assume for now that the top of the stack is the largest number. Since
we don't have loops, we just have an op that we repeat seven - 1 times.

The op is, on a high-level view: compare the current two topmost
elements on the stack and keep the higher of the two. 

However, < and > destroy both operands, whereas we want to destroy only
the lower of them. So first, we duplicate both operands, using 'over';
then we compare and conditionally keep the higher of the two, a la: )

``<{'_}{_}_

( Note that '<' is Stack(SP - 1) < Stack(SP); thus, if the topmost word
is higher, we swap the two words and drop; else we just drop. Either
way, the lowest of the two is derped. )

( Now we just repeat this five more times (six comparisons in total). )

``<{'_}{_}_ ``<{'_}{_}_ ``<{'_}{_}_ ``<{'_}{_}_ ``<{'_}{_}_

( And we print. )

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