Time and Concurrency
 
 
Tokens take time in transitions
- transition specified as having cost or delay
 - value can come from token and/or transition
 - mean and distribution given (fixed, exponential, Hurst)
 - must have sufficient non-null transitions to progress
 
any number of tokens can be present in a transition concurrently
- passage of tokens is independent - overtaking
 
all simulation questions are in terms of duration of some transition instance