#	PDCO
#
#	Programmer-defined control operations
#
#	Ralph E. Griswold and Michael Novak
#
#	Last modified 8/11/84
#

procedure Alt(a)
   local x
   while x := @a[1] do suspend x
   while x := @a[2] do suspend x
end

procedure Colseq(a)
   suspend |@!a
end

procedure Comseq(a)
   local x1, x2
   while x1 := @a[1] do
      (x1 === @a[2]) | fail
   if @a[2] then fail else return *a[1]
end

procedure Cond(a)
   local i, x
   every i := 1 to *a do
      if x := @a[i] then {
         suspend x
         suspend |@a[i]
         fail
         }
end

procedure Every(a)
   while @a[1] do @^a[2]
end

procedure Galt(a)
   local e
   every e := !a do suspend |@e
end

procedure Lcond(a)
   local i
   every i := 1 to *a by 2 do
      if @a[i] then {
         suspend |@a[i + 1]
         fail
         }
end

procedure Limit(a)
   local i, x
   while i := @a[2] do {
      a[1] := ^a[1]
      every 1 to i do
         if x := @a[1] then suspend x
         else break
      }
end

procedure Ranseq(a)
   local x
   while x := @?a do suspend x
end

procedure Repalt(a)
   local x
   repeat {
      while x := @a[1] do suspend x
      if *a[1] = 0 then fail
      else a[1] := ^a[1]
      }
end

procedure Resume(a)
   local i
   while i := @a[2] do {
      a[1] := ^a[1]
      every 1 to i do if @a[1] then @^a[3] else break
      }
end

procedure Select(a)
   local i, j, x
   j := 0
   while i := @a[2] do {
      while j < i do
         if x := @a[1] then j +:= 1
         else fail
      if i = j then suspend x
      else stop("selection sequence error")
      }
end