- stretch drag (using shift on the curve) operates on
the true meaning of constant. in other words,
the only way you can make a tempo ramped is by adjusting the end
tempo (shift drag on the *next* mark). from this point onwards.
shift-drag the curve will change the start tempo and ctrl-drag
the curve will change the end (needs work). to reset the
curve to constant, 'make constant' should simply set the end
tempo (unimplemented).
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