Mixed logit model

Hello everyone
I am analyzing my data by using m.logit package in R to create a mixed logit model. I conducted one mixed logit model with generic variables with the lognormal distribution. i have three altervatives: Car, PT, AV
here is the code mixedlogit1= mlogit(Choice ~TT +TC+SFF+COMF|1 ,id="ID",data=trainresh ,rpar=c(TC="ln",TT="ln"),
R=100, halton=NA, print.level=0, correlation = FALSE,panel=TRUE).

now I am wondering about how to create a mixed logit model with travel time (TT) as an alternative specific variable with alternative specific coefficients. that means each alternative has a specific time coefficient.

thank you in advance
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