Computes `tlag`

from uncorrected fit to the beta
exponential function. The name `tlag`

is a misnomer; it simply
is the maximum of the PDR curve, so in papers by Bluck et al. it is renamed to t_max.

Maes B D, Ghoos Y F, Rutgeerts P J, Hiele M I, Geypens B and Vantrappen G 1994 Dig. Dis. Sci. 39 S104-6.

## Arguments

- cf
named vector of coefficients; only

`k`

and`beta`

are required`k`

is measured in 1/min (e.g. 0.01/min).

## See also

`exp_beta`

, and `t50_bluck_coward`

for an example.