fitResult 
Monotherapy (onaxis) model fit, e.g. produced by
fitMarginals . It has to be a "MarginalFit" object or a
list containing df , sigma , coef ,
shared_asymptote and method elements for, respectively,
marginal model degrees of freedom, residual standard deviation, named
vector of coefficient estimates, logical value of whether shared asymptote
is imposed and method for estimating marginal models during bootstrapping
(see fitMarginals ). If biological and power transformations
were used in marginal model estimation, fitResult should contain
transforms elements with these transformations. Alternatively, these
can also be specified via transforms argument.

null_model 
Specified null model for the expected response surface.
Currently, allowed options are "loewe" for generalized Loewe model,
"hsa" for Highest Single Agent model, "bliss" for Bliss additivity,
and "loewe2" for the alternative Loewe generalization.

transforms 
Transformation functions. If nonnull, transforms is
a list containing 5 elements, namely biological and power transformations
along with their inverse functions and compositeArgs which is a list
with argument values shared across the 4 functions. See vignette for more
information.
