# Read a text file of SubX segment 'fragments' with duplicate names, and emit
# a list of 'merged' segments.
#
# Example input:
#   == A
#   a
#   b
#   c
#   == B
#   d
#   e
#   == A
#   f
#   g
#   == A
#   h
#
# Output:
#   == A
#   h
#   f
#   g
#   a
#   b
#   c
#   == B
#   d
#   e
#
# The output gives each segment the contents of all its fragments, with later
# fragments *prepended* to earlier ones.
#
# Prepending necessitates buffering output until the end. We'll convert
# fragments to distinct streams, maintain each segment as a linked list of
# fragments that's easy to prepend to, and finally emit the linked lists in
# order.