smaller of 2 first indexes

#Return the smaller of x[0] and y[0]
#only return nil if both y[0] and y[0] are nil
#x and y may possibly be empty arrays
def min_first_element x, y
  if x[0] and y[0] #both are non-empty
    [x[0],y[0]].min
  elsif x[0]
    x[0]
  elsif y[0]
    y[0]
  else
    nil
  end
end