$chan.ison
Checks if a user is on a channel
Usage
<boolean> $chan.ison(<nickname:string>[,<window_id:string>])
Description
Returns 1 if <nickname> is on the channel identified by <window_id>, and 0 otherwise.
If <window_id> is not specified the current window is used (assuming that it is a channel at all).
If the window is not a channel, a warning is printed and an empty string is returned.

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