Asymptote/Command/plain/Relative

side Relative(explicit pair align);

side Leftside= Relative(W);
side Center=Relative((0,0))
side RightSide=Relative(E);

position Relative(real position);

The position Relative(real) specifies a location relative to the total arclength of the path. These convenient abbreviations are predefined:

position BeginPoint=Relative(0);
position MidPoint=Relative(0.5);
position EndPoint=Relative(1);