def click_estimate_curvature():
c1 = _pylab.ginput()<EOL>if len(c1)==<NUM_LIT:0>:<EOL><INDENT>return None<EOL><DEDENT>c2 = _pylab.ginput()<EOL>if len(c2)==<NUM_LIT:0>:<EOL><INDENT>return None<EOL><DEDENT>return <NUM_LIT:2>*(c2[<NUM_LIT:0>][<NUM_LIT:1>]-c1[<NUM_LIT:0>][<NUM_LIT:1>])/(c2[<NUM_LIT:0>][<NUM_LIT:0>]-c1[<NUM_LIT:0>][<NUM_LIT:0>])**<NUM_LIT:2><EOL>
Takes two clicks and returns the curvature, assuming the first click
was the minimum of a parabola and the second was some other point.

Returns the second derivative of the function giving this parabola.

Right-click aborts.