Function push_call 

pub fn push_call(
    chain_id: &str,
    entry_key: &str,
) -> Result<CallChainGuard, String>

Push entry_key onto chain_id’s call stack, detecting cycles.

Call this before dispatching a callback for entry_key under chain_id; hold onto the returned guard for the duration of the dispatch so entry_key is popped back off when it completes, however it completes. Returns Err describing the current chain if entry_key is already on the stack - the same capability being invoked again before an earlier invocation of it has returned, i.e. a cycle - and dispatch should not proceed.