Ad hoc appointments: temporary or chance-based roles differ from formal appointments when qualifications and continuity exist. The terms ad hoc, stop-gap and fortuitous differentiate temporary or accidental appointments from formal ones: ad hoc denotes purpose-specific formation, stop-gap denotes temporary remedial measures, and fortuitous denotes events occurring by chance; an appointment made after due consultation and with prescribed qualifications that endures for a substantial period will not ordinarily be regarded as fortuitous.