Note that this reference documentation is identical to the help that is displayed in MATLAB when you type “help ft_checkopt”.

  FT_CHECKOPT does a validity test on the types and values of a configuration
  structure or cell-array with key-value pairs.
  Use as
    opt = ft_checkopt(opt, key)
    opt = ft_checkopt(opt, key, allowedtype)
    opt = ft_checkopt(opt, key, allowedtype, allowedval)
  For allowedtype you can specify a string or a cell-array with multiple
  strings. All the default MATLAB types can be specified, such as
  Furthermore, the following custom types can be specified
    'doublebivector'             i.e. [1 1] or [1 2]
    'ascendingdoublevector'      i.e. [1 2 3 4 5], but not [1 3 2 4 5]
    'ascendingdoublebivector'    i.e. [1 2], but not [2 1]
  For allowedval you can specify a single value or a cell-array
  with multiple values.
  This function will give an error or it returns the input configuration
  structure or cell-array without modifications. A match on any of the
  allowed types and any of the allowed values is sufficient to let this
  function pass.