The standard deviation is calculated as the square root of the diagonal of the variance coavariance matrix of your random parameters. The variance- covariance matrix is calculated using the estimated lower Cholesky matrix.
fn_se_std(a, L, lambda)An integer indicating the row position of the random parameter in the asymptotic variance-covariance matrix
The lower cholesky matrix
The asymptotic variance covariance matrix
A vector of standard errors
http://www.stephanehess.me.uk/software.html Daly, A. J., Hess, S., & de Jong, G., 2012, Calculating errors for measures derived from choice modelling estimates, Transportation Research Part B, 46(2):333-341