GATHER - A collection of traces sorted (or gathered) according to some
         criteria such as "shot gather" or "RP" gather.  SIOSEIS
         frequently refers to "a gather" meaning rp sorted, whereas
         a "shot" means a shot sorted.

RP  - Reflection Point or CDP (common depth point) or CMP (common
CMP   mid point) gather.  SIOSEIS uses SEG-Y trace header word 7 as
CDP   an indicator that the data are sorted by RP (reflection point).
       An SIOSEIS rp sorted gather is terminated by using a special
      SEG-Y trace header flag (long integer word 51 set to -1), or a
      change in the RP number (header word 6) when using parameter
      FORGAT (foreign gather).

SHOT - The collection of traces associated with the field shot.  SEG-Y
       trace header word 4 (shot record number) is used for the "shot
       number".  SEG-Y word 7 (rp trace number) must be 0 to be 
       considered shot sorted.

FNO/LNO - First/Last number, where number is either a shot or rp, depending
       on how the data are sorted.

FNO-LNO List - Many seismic processes require parameter values to change
       on different shots/rps or traces.  Each fno/lno list must be
       terminated by the word END (case insensitive).  e.g.
            fno 39987 twp 17 0 22 0 23 0 24 0  end
            fno 39989 twp 1 0 17 0  end
            fno 39990 twp 17 0 18 0  end

PRESET - The parameter value stays the same from fno/lno list to list
         until specified or given again.

DEFAULT - The parameter value reverts back to the original value after
          every fno/lno list.