Is your feature request related to a problem? Please describe.
There is an optional field in stop_times.txt timepoint that is used to specify a timepoint that a transit vehicle should strictly adhere to (e.g., hold if they are running early). If a trip is defined in frequencies.txt with the exact_times=0 value, then it is a frequency-based trip, which means that a schedule doesn't exist.

Therefore, frequency-based trips with exact_times=0 should NOT have any timepoint=1 values in stop_times.txt - the field should either be 0 or blank.

Proposed solution
New rule - if a GTFS dataset has stop_times.timepoint=1 for any trips in frequencies.txt with exact_times=0, that is an error.