[PyMOL] Bug report: cmd.fab() with existing object name produces unexpected results and downstream segfault

[Sampson, J. <Jar...@ny...>]

In the process of investigating Ben Bobay's question<https://sourceforge.net/p/pymol/mailman/message/34128680/> about keyboard shortcuts for adding a methyl group, I may have found a bug, or at least unexpected behavior, in cmd.fab().

When first creating a peptide, it works fine:

    fab AAAA, pep

However, if I call it again with a different (or the same) sequence,

    fab DDDD, pep

The new residues are overlaid right on top of the initial ones, and interspersed in the sequence order (overlapping sticks, Screenshot1<https://www.dropbox.com/s/e6hmr0vb8jlfq6q/Screenshot%202015-05-20%2013.00.36.png?dl=0>).  This breaks cartoon representation, as Ala1 and Ala2 are no longer contiguous in the sequence (`as cartoon` leaves nothing drawn, Screenshot2<https://www.dropbox.com/s/p85bsftrgi54pq3/Screenshot%202015-05-20%2013.00.49.png?dl=0>).

Also, something happens when switching representations (e.g. from cartoon to sticks) that segfaults with certain sequences.  I can semi-reproducibly produce a segfault (perhaps 2/3 of the time) with the following commands in Open Source PyMOL (1.7.4.0), PyMOLX11Hybrid (1.7.6.0), and MacPyMOL (1.7.6.0).

    reinitialize
    fab AAAA, pep
    fab ADAA, pep
    as cartoon
    as sticks
    # sometimes it takes an extra line or two
    #fab AADA, pep
    #as sticks

I believe this happens specifically when some residues are the same and some are different: I haven't been able to produce a segfault using "DDDD" as the second sequence, but adding the third sequence (second to last line) reintroduces the segfault.

I would expect calling `fab` with an existing object name to perform one of the following actions (from most to least preferable):

1. Append the residues to the C-terminus of the existing object with sensible geometry, similar to `editor.attach_fragment()`.
2. Add a new state to the object with the new sequence, as with `cmd.load(state=0)`.
3. Overwrite the existing object (probably not a good idea).

It may also be helpful to have additional arguments to fab, such as append=1 or state=0, depending on which of the above options might be implemented.

I'm adding this as a bug report on SourceForge as well.

Cheers,
Jared