Clears the "AST active" status.

    This enables delivery of asynchronous system traps (ASTs) for the
    access mode from which the service call was issued, while an AST
    routine is active.

                                   NOTE

       The explicit use of $CLRAST is strongly discouraged, as
       it complicates synchronization issues and may lead to the
       unbounded consumption of stack space.

    Normally, AST delivery for a particular access mode is deferred
    while an AST routine is executing in that access mode. When the
    AST routine returns, an implicit call is made to $CLRAST to re-
    enable AST delivery.

    Explicitly calling $CLRAST within an AST routine allows the
    delivery of ASTs for the access mode from which the service call
    was issued, prior to completion of the active AST routine.

    Format

      SYS$CLRAST