code: def joinMeiUyir(mei_char, uyir_char):
      
      if not mei_char: return uyir_char
      if not uyir_char: return mei_char
      
      if not isinstance(mei_char, PYTHON3 and str or unicode):
          raise ValueError(u"Passed input mei character '%s' must be unicode, not just string" % mei_char)
      if not isinstance(uyir_char, PYTHON3 and str or unicode) and uyir_char != None:
          raise ValueError(u"Passed input uyir character '%s' must be unicode, not just string" % uyir_char)
      if mei_char not in grantha_mei_letters:
          raise ValueError(u"Passed input character '%s' is not a tamil mei character" % mei_char)
      if uyir_char not in uyir_letters:
          raise ValueError(u"Passed input character '%s' is not a tamil uyir character" % uyir_char)
      if uyir_char:
          uyiridx = uyir_letters.index(uyir_char)
      else:
          return mei_char
      meiidx = grantha_mei_letters.index(mei_char)
      # calculate uyirmei index
      uyirmeiidx = meiidx*12 + uyiridx
      return grantha_uyirmei_letters[uyirmeiidx]
docs: This function join mei character and uyir character, and retuns as
      compound uyirmei unicode character.
  
      Inputs:
          mei_char : It must be unicode tamil mei char.
          uyir_char : It must be unicode tamil uyir char.
  
      Written By : Arulalan.T
      Date : 22.09.2014