[R] qualitative response model

[Z Eric]

Hi, I want to know is there other functions in R to
estimate qualitative response model besides multinom()
in library nnet, if this is the only possibility, I
have a question about the application:
for example:
there is three transportation choice : car, bus ,
subway.
each alternative has own characteristic variables,
I want to apply conditional logit model to analysis
the choice of three alternatives, but if some cases
face different choice set, i.e only car and bus are
available, how to use this function?
I am a newcomer in R, hope the simple question not to
bother you.
thanks for the help.