The reason is that we iterate over the atoms and add the per atom
accessibility to the current value. You can guess what happens when
calling the Accessibility function several times...

structure of single residues

This changes behaviors in MatchResidueByLocalAln and MatchResidueByGlobalAln (
or Superpose with match = local-aln or global-aln) which previously failed for
non-"protein" chains (i.e. anything w/o peptide bonds). Now, amino acid chains
without peptide bonds (e.g. CA-only) are acceptable and other chains (e.g.
ligands / water) are skipped correctly.

BOOST_MESSAGE has been obsolete since boost 1.34 (2007). So after 10 years, we
can finally get rid of it...