[fitgcvtn] Systematic element digit letters

Systematic element names, in particular the initial letters of their components used to form the systematic element symbol, establish a one-to-one mapping between letters and digits:

0123456789 = nubtqphsoe

This is better than many other digit-letter systems which face a difficulty of whether the first letter should stand for 0 or for 1.  For example, all of these are reasonable.

0123456789 = abcdefghij
0123456789 = jabcdefghi
0123456789 = zabcdefghi

You just have to know which convention is being used.  Similar issues arise with the CHARLESTON code.

In contrast, with the systematic element code, zero = null/nihil (N) is encoded into the meaning of the letter.

Possible difficulties with this Greek and Latin derived code:

Need to remember that 1 is U not mono (M).
Need to remember that 2 is B not di (D).
Need to remember that 67 is hex(H)-sept(S), not sex(S)-hept(H).

Because 3 is unambiguously T in both languages, it forces 4 not to be tetra, so 4 must be Q (quattuor).  This forces 5 not to be quinque, so 5 must be P (pente).  It's nice that things work out.  However, reconstructing this line of logic takes a bit of thought.