for non-standard things in the check directory

Found the following math rendering problems:
parfm.Rd:112-113: Undefined control sequence: \slash in ...exp \left( - u \slash \theta \right)}...
Found the following files/directories:
  'NUL'
Found the following math rendering problems:
parfm.Rd:112-113: Undefined control sequence: \slash in ...exp \left( - u \slash \theta \right)}...
Found the following files/directories:
  ‘NUL’
Found the following files/directories:
  ‘NUL’
Found the following files/directories:
  ‘NUL’
Found the following files/directories:
  ‘NUL’
Found the following files/directories:
  ‘NUL’