seven slash display (stdout version)

import os, terminal

iterator digit(c: char): string =
  var i = 0
  while i < 3:
    case i:
    of 0:
      case c:
      of '0', '2', '3', '7', '8', '9': yield "/\\"
      of '1', '4': yield " \\"
      of '5', '6': yield "/"
      else: yield ""
    of 1:
      case c:
      of '0': yield "\\ \\"
      of '1', '7': yield "  \\"
      of '2', '-': yield " /"
      of '3': yield " /\\"
      of '4', '5', '6', '8', '9': yield "\\/\\"
      else: yield ""
    of 2:
      case c:
      of '0', '2', '6', '8': yield " \\/"
      of '3', '5', '9': yield "  /"
      else: yield ""
    else: discard
    inc i

proc display(s: string) =
  var
    offset = (s.len)*2
    index = s.high
  if s[0] notin {'-', '0'..'9'}:
    echo "Error: Not a number."
    return
  for c in s[1..^1]:
    if c notin {'0'..'9'}:
      echo "Error: Not a number."
      return
  while index >= 0:
    for line in digit(s[index]):
      terminal.setCursorXPos(offset)
      echo line
    terminal.cursorUp()
    dec index
    offset -= 2

if paramCount() > 0:
  display(paramStr(1))