FixHydrogenAngles(done[],atm,lastatm)
{ // 'done' is a table that flags atoms which have already been analyzed.
  // The analysis starts at 'atm' and should not recurse to 'lastatm'.

  // First make sure that each atom is analyzed only once:
  if (done[atm]) return
  done[atm]=1

  // Maybe we will recurse to 'nextatm', but not yet
  nextatm=NONE

  // Store the bound hydrogens in 'hydtab' and their number in 'hydrogens'
  GetBoundHydrogens(hydtab,&hydrogens,atm)

  if (hydrogens==4)
  { // Handle four hydrogens like methane with two constraints
    FixAngle(hydtab[0],atm,hydtab[1])
    FixAngle(hydtab[2],atm,hydtab[3]) }

  if (hydrogens==3)
  { // Handle three hydrogens like CH3,NH3 groups with three constraints
    FixAngle(hydtab[0],atm,hydtab[1])
    FixAngle(hydtab[1],atm,hydtab[2])
    FixAngle(hydtab[2],atm,hydtab[0])
    // Don't recurse if the heavy atom bound to atm is sp3 with <=1 hydrogens
    nextatm=BoundHeavyAtom(atm)
    if (nextatm!=NONE and BoundHydrogens(nextatm)<=1 and Bonds(nextatm)>3) nextatm=NONE }

  if (hydrogens==2)
  { if (Bonds(atm)>3)
    { // Two hydrogens bound to sp3 atom. These need constraints to the next heavy atom:
      nextatm = A heavy atom bound to atm, which is not lastatm, which has <3 hydrogens,       
                which does not already have an angle constraint atm-nextatm-x,         
                and which has the highest score. The score is 3 if nextatm has two bound
                hydrogens (it's best if we continue along a -CH2- chain), 2 if it has 0   
                hydrogens (but also OK to end at an atom without hydrogens), 1 if it has
                1 hydrogen and 3 bonds, otherwise 0. }
    if (nextatm!=NONE)
    { // Found a bound heavy atom that can take the constraints
      FixAngle(hydtab[0],atm,nextatm)
      FixAngle(hydtab[1],atm,nextatm)
      // Again, don't recurse if nextatm is sp3 with <=1 hydrogens, these are handled later
      if (BoundHydrogens(nextatm)<=1 and Bonds(nextatm)>3) nextatm=NONE }
    else
    { // Two hydrogens bound to sp2 atom (or no nextatm found), these can safely be coupled
      FixAngle(hydtab[0],atm,hydtab[1]) } }

  if (hydrogens==1)
  { // A single hydrogen gets a single constraint. Find best partner atom. 
    nextatm = A heavy atom bound to atm, which has <3 hydrogens and the best score.
              The score is -(number of bound hydrogens + number of angle constraints
              nextatm-x-y)*4. If nextatm==lastatm or there already is an angle constraint
              atm-nextatm-x, then score=score-20. If nextatm is not in the same residue
              as atm, then score=score-10. 
    if (nextatm!=NONE) FixAngle(hydtab[0],atm,nextatm) }
  // Recurse if it makes sense
  if (nextatm!=NONE) FixHydAngles(done,nextatm,atm) }