count1=4 countdefcolumns=1
defsubpre{
  & ##
  advancecolumns by -1 ifnumcolumns>0 spansubpre fi
}
defpreamb{
  ##
  advance columns by -1
  thecolumns
  ifnumcolumns>0 spansubpre fi
  cr
}
halign{spanpreamb
  first line cr
  second line cr
  third line cr
}
bye

! TeX capacity exceeded, sorry [input stack size=5000].
subpre ...by -1 ifnum columns >0 span subpre
                                                  fi
subpre ...by -1 ifnum columns >0 span subpre
                                                  fi
subpre ...by -1 ifnum columns >0 span subpre
                                                  fi
subpre ...by -1 ifnum columns >0 span subpre
                                                  fi
subpre ...by -1 ifnum columns >0 span subpre
                                                  fi
subpre ...by -1 ifnum columns >0 span subpre
                                                  fi
...
l.13 halign{spanpreamb