title: Tcl: Interaction of extension loading and threads
body: <p>Can several instances of a Tcl_PackageInitProc() be executing
  concurrently (in different threads), if TCL was configured
  for threads?</p>
  
  <p>For reasons of backward compatibility, I believe
  invocations of the initialization and unload procedures must be
  serialized.</p>
  
  <p>The manual is silent on the behavior: are invocations of these routines
  serialized, or must extension writers deal with synchronization, in
  particular mutual exclusion, in these routines?</p>