Locate a trailing truncated copy of the localized reader label and
return the index at which the description body ends (i.e. where the
partial label begins). Returns -1 when no partial label is present or
when the label is present in full (a complete clause we keep).
Locate a trailing truncated copy of the localized reader label and return the index at which the description body ends (i.e. where the partial label begins). Returns -1 when no partial label is present or when the label is present in full (a complete clause we keep).