asdf.us/ascii tips
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These keyboard commands work in brush mode (square, circle, cross):

[                          brush smaller
]                          brush bigger
ctrl~click on brush        erase cell
ctrl~click on canvas       draw with bg color
shift~click on canvas      draw line from last position
alt~click on canvas        fill brush with sampled color
alt~shift~click on canvas  copy canvas to brush
rightclick on palette      set bg color (when drawing with a letter)

h/t timb for guide