Minimum distance tetrad

Here's a nice formula which follows from the fact that the 7-limit
diamond is [-1..0,-1..0,-1..0]. If [a,b,c] is the ontonal tetrad
corresponding to some interval 1 <= q < 2, then the tetrad with
minimal distance to the origin tetrad is well defined. For each of a,
b, and c, substract 1 if they are positive, and leave unchanged if
they are negative; the resulting tetrad is the one closest to the
origin containing q.

For instance, for 225/224 we have [1,1,4] as the otonal root tetrad,
and so [0,0,3] as the minimal tetrad. This can be a useful thing to
know for scale construction.