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);