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

  FT_PREAMBLE is a helper function that is included in many of the FieldTrip
  functions and which takes care of some general settings and operations at the
  begin of the function.
  This ft_preamble m-file is a function, but internally it executes a
  number of private scripts in the callers workspace. This allows the
  private script to access the variables in the callers workspace and
  behave as if the script were included as a header file in C-code.