rotations live in planes — 6 in 4-D = n(n−1)/2:
x∧y · x∧z · y∧z · x∧t · y∧t · z∧t
DOUBLE = one angle applied to two orthogonal planes (a self-dual pair) — a motion with no 3-D counterpart.
5 vertices · 10 edges · 10 triangles · 5 tetrahedral cells. The pentachoron is self-dual: every vertex is the apex of a unique cell, opposite it.
FILLS: cell fills = the 5 tetrahedra, one hue each; face fills = the 10 triangles, shaded by 4-D depth.
4-D PERSP: eye at (0,0,0,D) on the t-axis, f = D/(D−t) — the near cell really foreshortens.
FLAT-87: plots (x,y) only, lines exactly as TESSERAC.BAS did — the 1987 projection.