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| $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. |