function [t] = p_intersect(e,v,p,n)
%Ax + By + Cz - D == 0 is solved simultaneously
%with e + t*v, for t.
D = n*p';
numerator = D - n*e';
denomenator = n*v';
if denomenator == 0
    t = -10;
else
    t = numerator/denomenator;
end