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import types
namespace BOOT

-- Dequeue functions
 
-- dqUnit makes a unit dq i.e. a dq with one item, from the item
 
-- dqUnitCopy copies a unit dq
 
-- dqAppend appends 2 dq's, destroying the first
 
-- dqConcat concatenates a list of dq's, destroying all but the last
 
-- dqToList transforms a dq to a list
 
dqUnit s==
  a := [s]
  [a,:a]
 
dqUnitCopy s== dqUnit(CAAR s)
 
dqAppend(x,y)==
    if null x
    then y
    else if null y
         then x
         else
              x.rest.rest := first y
              x.rest := rest y
              x
 
dqConcat ld==
    if null ld
    then nil
    else if null rest ld
         then first ld
         else dqAppend(first ld,dqConcat rest ld)
 
dqToList s==if null s then nil else first s
 
dqAddAppend(x,y)==
    if null x
    then nil
    else if null y
         then nil
         else
              x.rest.rest := first y
              x.rest := rest y
              x