code: def to_haystack(unit):
      
      unit = str(unit)
      global HAYSTACK_CONVERSION
      global PINT_CONVERSION
      if unit == 'per_minute' or \
          unit == '/min' or \
          unit == 'per_second' or \
          unit == '/s' or \
          unit == 'per_hour' or \
          unit == '/h' or \
          unit == None:
          return ''
          # Those units are not units... they are impossible to fit anywhere in Pint
      
      for pint_value, haystack_value in PINT_CONVERSION:
          unit = unit.replace(pint_value, haystack_value)
      for haystack_value, pint_value in HAYSTACK_CONVERSION:
          if pint_value == '':
              continue
          unit = unit.replace(pint_value, haystack_value)
      return unit
docs: Some parsing tweaks to fit pint units / handling of edge cases.