[PyMOL] superposition of two molecules

[William S. <wg...@ch...>]

  This is driving me nuts.  I am trying to superimpose two RNA molecules:


PyMOL>load trna10.pdb
....
Executive: object "trna10" created.
PyMOL>load ref4.pdb
....
  Executive: object "ref4" created.
PyMOL>color blue
  Executive: Colored 2668 atoms.
PyMOL>color yellow, ( trna10 and resid 10:15 and name P )
  Executive: Colored 6 atoms.
PyMOL>color red, ( ref4 and resid 10:15 and name P )
  Executive: Colored 6 atoms.

All of the above works without incident, so the atom selection syntax 
must be ok.

Then I want to fit the two sets of colored phosphorus atoms, selecting 
with the same syntax:


PyMOL>fit  ( trna10 and resid 10:15 and name P ), ( ref4 and resid 
10:15 and name P )
ExecutiveRMS-Error: No atoms selected.

same error if I replace fit with rms

What am I doing wrong?