FieldGenCentMat( module )
This function should only be applied if the function
IsIrreducible(module) has returned true, and if
IsAbsolutelyIrreducible(module) has returned false. A matrix which
centralises the G-module module and which has multiplicative order
q^e-1, where q is the order of the ground field and e is the
dimension of the centralising field of module, is calculated and
stored. It can be accessed as CentMatFlag(module).
GAP 3.4.4