The question was: "I got a problem using sketch commands. I'd like to make a tapeworm trace of phosphates for a tRNA. So I change my <molecule>_ca and <molecule>_ca_mxdst datablock. I can produce a Ca 'P' trace without any problem. But

        sk_ty tapeworm
        sk_set tapeworm solid 0.5
        sk_obj sktl1
        sk_add  d1 d76

I got the message 'Not enough main chain atom to form plane'. What is the problem ?"

Morten replied: "Unfortunately, tapeworm is hardwired for polypeptides, so it will not work for nucleic acids. You can use the rattler instead. You have to set the <mol>_molecule_ca datablock to be P or C4' or something first."