WITNESS - Solution to the Infocom adventure.

Go North twice to the front door and ring the bell. Phong will let you in.
Then just try to go East, and Phong will lead you to the Living Room.
Wait, and Linder will eventually take you to his office. Sit down in the
wooden chair. Keep waiting. Eventually, the murder will occur. Stand up,
then push the button. Examine the clock and the keyhole.

Get the powder analyzed. Examine the window. Get the green wire.
Go to the Butler's Room, read the mystery book. Get the gun receipt.
Go to Monica's room, unlock and open the back door. Go East to the Backyard.
Go South to the office path - get the muddy gun.
Go West to the Side Yard - check footprints.
Go to the Garage, then the Workshop - examine spool (matches green wire).

Wait for Monica. Follow her to her room. Ask her about Mr. Linder.
Follow her to the office. Handcuff her. Search Monica for the key.
Unlock the clock and open it. Search her for the gun.
Wait until Duffy returns and arrest her for the murder.