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# We could return the packstring to the user, as the representation of their
# callstack.  Instead, we choose to employ the same strategy of hash-key
# storage with unique-integer identifier as value.  The 'callstacks'
# attribute is a hash that holds callstack packstrings as keys.  This means
# that the user is given a simple integer to track their callstack.
#
# There are two other reasons, besides binary avoidance at the user-level,
# for mapping packstrings with the Callstacks hash.  Repeated callstacks
# happen whenever capturing is requested from a loop.  By mapping callstacks
# we only have to store a repeated callstack once.  We can also help the user
# identify loop iterations by offering storage for an 'annotation' value.
#
# If the user provides an annotation then we will need to save a copy of it,
# which we do pushing it onto an array - the 'annotations' attribute.  The
# index of the new element becomes the annotation's id.
#
# We can't use the same (hash) strategy employed for components and pack
# strings because annotation values can be any kind of scalar including
# references and undefs; things that don't survive stringification needed for
# storage as hash keys.
#