Subprograms using Marlin ? (no replies)

I am trying to make a filled square using Marlin, and it is/was my understanding that it can be implemented using some code similar to this:

G90
G0 X0 Y0
G1 X20 Y0
M98 P101
M98 P101
M98 P101
M98 P101
M98 P101
M98 P101
M98 P101
M98 P101
M98 P101
M98 P101
M30

O101
G91
G1 X0 Y1
G1 X-20 Y0
G1 X0 Y1
G1 X20 Y0
G90
M99

but this does not work. Is this an issue with Marlin? Is there any other easy way do do the same thing?