code: def valuecount(table, field, value, missing=None):
      
  
      total = 0
      vs = 0
      for v in values(table, field, missing=missing):
          total += 1
          if v == value:
              vs += 1
      return vs, float(vs)/total
docs: Count the number of occurrences of `value` under the given field. Returns
      the absolute count and relative frequency as a pair. E.g.::
  
          >>> import petl as etl
          >>> table = [['foo', 'bar'],
          ...          ['a', 1],
          ...          ['b', 2],
          ...          ['b', 7]]
          >>> etl.valuecount(table, 'foo', 'b')
          (2, 0.6666666666666666)
  
      The `field` argument can be a single field name or index (starting from
      zero) or a tuple of field names and/or indexes.