(50g) Fun with Farey sequences

[Tugdual]

A video to start with.
A wikipedia page for the algorithm.
And some code for the 50g

Code:

%%HP: T(3)A(R)F(,);
\<< 0 1 1 0 1 \-> N A B C D nb
  \<< N 'D' STO A B /
    WHILE C N <
    REPEAT 'nb' INCR DROP N B + D / IP R\->I DUP C * A - SWAP D * B - C 'A' STO D 'B' STO 'D' STO 'C' STO A B /
    END nb \->LIST
  \>>
\>>

Usage: enter order N and call the function; returns list of items for the Farey sequence order N.