%!TEX TS-program = sketch

% vertices of the tetrahedron
def p1 (0,0,1) def p2 (1,0,0)
def p3 (0,1,0) def p4 (-.3,-.5,-.8)

% faces of the tetrahedron.
polygon(p1)(p2)(p3) % original front polygon
polygon(p1)(p4)(p2) % bottom
polygon(p1)(p3)(p4) % left
polygon(p3)(p2)(p4) % rear
% line to pierce the tetrahedron
line[linecolor=red](-1,-1,-1)(2,2,2)

global {language tikz}
