nomcompile3: Build a set of nomenclature files

# - - -   T a x o n . c h i l d K e y

    def childKey ( self, childRank ):
        '''What .shortTxKey will the next child have?
        '''

The child's short key consists of the concatenation of: the parent's short key; zeroes for any missing ranks between the parent and child rank; and the child's birth order plus one, left-zero-padded to the correct length for the child's rank. We'll build a list of these pieces, then concatenate them to return the result.

        #-- 1 --
        result = [ self.shortTxKey ]

For the function that returns the key length for each rank, see Section 15.7, “Hier.keyLen(): Find the key length for a given depth”.

        #-- 2 --
        # [ result  +:=  (strings containing N zero digits, where
        #       N is the sum of the key lengths for all ranks
        #       between self.rank and childRank) ]
        for deepx in range ( self.rank.depth+1, childRank.depth ):
            result.append ( '0' * self.hier.keyLen(deepx) )

        #-- 3 --
        # [ result  +:=  (len(self)+1) as a string, zero-padded
        #                to the key length for childRank ]
        result.append ( str(len(self)+1).rjust ( childRank.keyLen,
                                                 '0' ) )

        #-- 4 --
        return ''.join(result)