A set of or classes for compatibility purposes. It is used to be able to identify null values.
callorNULLcall or NULL class.
characterorNULL character or null.
expressionorNULL expression or null.
fittmethodorNULL fittmethod or null.
functionorNULL function or null.
integerorNULL integer or null.
listorNULL list or null.
logicalorNULL logical or null.
matrixororNULL matrix or null.
nl.fitt.roborNULL nl.fitt.rob or null.
fittorNULL fitt or null.
nl.formorNULL nl.form or null.
nl.numericorNULL numeric or null.
vectororMatrix vector or Matrix.
vectororNULL vector or null.