NB. dfridr - Derivative by Ridder;s method

CON =: 1.4
CON2=: *:CON
BIG =: 1e30
NTAB=: 10
SAFE-: 2.0

NB. Usage 'df err' =: f(y) dfridr X ; h
NB. where f(y) is a char string function name or
NB. string equation in terms of y.
NB. Return the derivative evaluated at X.
dfridr =: 4 : 0
f =. 3 : x
X=. >{.y
h=. >}.y

if. h=0.0 do. 
	'h must be nonzero in dfridr'
	exit
end.

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