proc dist { a b } {
  # Distance between two vectors
  return [veclength [vecsub $b $a]]
}

proc angle { a b } {
  # Angle between two vectors
  set amag [veclength $a]
  set bmag [veclength $b]
  set dotprod [vecdot $a $b]
  return [expr 57.2958 * acos($dotprod / ($amag * $bmag))]
}

proc fsel { sel1 sel2 cutoff } {
  set n [molinfo top get numframes]
  for {set i 0} {$i < $n} {incr i} {
    set sel [atomselect top "(within $cutoff of $sel1) and $sel2" frame $i]
    set num [$sel num]
    set resid [lsort -unique [$sel get {resid}]]
    puts "$i $resid"
  }
}