help function to keep slope and intercept portion ready in mixed model expression
get_slope_intercept_cross(
expression,
random_intercept_vars,
intercept_vars_pairs,
random_slope_intercept_pairs,
uncorrel_slope_intercept_pairs
)
expression expression created
expression created so far
names of variables for random intercept
those of the random intercept variables with nested effect
random slopes intercept pairs this is a list of paired variables
variables with correlated intercepts