rebol [] ;** R3 only

s:  {
  1  4         5  8
                        11

  3  2         6  7  9  10
}

_:[skip]r:[(z: 77 + n // 81)z _ _ then[[i | j]reject | r | p:(p/1: pick"/|\-"n)end]|(++ n)]i: #"1"
parse s[any[79[" "| and lf insert" "| _ not" "p:(p/1: p/1 +(-48 + p/0 * 10)p/0:" ")| _]_]
99[:s(i:(j: i)+ n: 1)to[i | j]0 4[and r]to j p:(p/1:"+")]]print s

halt

>>
  +  +---------+  +
   \/          |  |\     +
   /\          |  | \    |
  +--+         +--+  +---+