ex15.md

numeric u; % 声明和定义不能同时

u := 2cm;
pair o,a,b;

o := (0,0)*u;
a := (1,0)*u;

draw o -- (1,0)*u;
b := dir 40;
draw o -- (b*u);

drawarrow (dir 0)*.3*u{up} .. (dir 40)*.3*u ;