old_contents: import numpy as np
  import numpy.ma as ma
  
  
  def _append_field(sta, data, dtype=None, position=None, masked=False):
  
      newdtype = sta.dtype.descr
      if np.equal(position, None):
          newdtype.append(dtype)
      else:
          newdtype.insert(position, dtype)
      newdtype = np.dtype(newdtype)
  
      if masked:
          newsta = ma.empty(sta.shape, dtype=newdtype)
      else:
          newsta = np.empty(sta.shape, dtype=newdtype)
  
      for field in sta.dtype.fields:
          newsta[field] = sta[field]
          if masked:
              newsta[field].set_fill_value(sta[field].fill_value)
  
      newsta[dtype[0]] = data
      if masked:
          newsta[dtype[0]].set_fill_value(data.fill_value)
  
      return newsta
  
  
  def _drop_fields(sta, names, masked=False):
  
      names = set(names)
  
      newdtype = np.dtype([(name, sta.dtype[name]) for name in sta.dtype.names
                         if name not in names])
  
      if newdtype:
          if masked:
              newsta = ma.empty(sta.shape, dtype=newdtype)
          else:
              newsta = np.empty(sta.shape, dtype=newdtype)
      else:
          return None
  
      for field in newdtype.fields:
          newsta[field] = sta[field]
          if masked:
              newsta[field].set_fill_value(sta[field].fill_value)
  
      return newsta
  
new_contents: import numpy as np
  
  
  def _append_field(table, data, dtype=None, position=None):
  
      newdtype = table.dtype.descr
      if position is None:
          position = len(newdtype)
  
      newdtype.insert(position, dtype)
      newtable = np.empty(table.shape, dtype=newdtype)
  
      for field in table.dtype.fields:
          newtable[field] = table[field]
  
      newtable[dtype[0]] = data
  
      return newtable
  
  
  def _drop_fields(table, names):
  
      names = set(names)
      newdtype = [(name, table.dtype[name]) for name in table.dtype.names
                  if name not in names]
      newdtype = np.dtype(newdtype)
  
      if newdtype:
          newtable = np.empty(table.shape, dtype=newdtype)
      else:
          return None
  
      for field in newdtype.fields:
          newtable[field] = table[field]
  
      return newtable
  
subject: remove support for masked arrays in structhelper.