pool() on lavaan fit objects gives error: coef() not available on S4 object

[Gootjes]

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Describe the bug The error happens in get.dfcom on this line https://github.com/amices/mice/blob/c924a7473ea2a95a551ca54479557877defb6423/R/get.df.R#L24
The package lavaan implements coef as an S4 function (compare lavaan::coef to stats::coef), however, this function is not available at this point, so the regular coef function is called on the lavaan fit object, leading to an error.

To Reproduce

library(lavaan)
#> This is lavaan 0.6-15
#> lavaan is FREE software! Please report any bugs.
library(mice)
#> 
#> Attaching package: 'mice'
#> The following object is masked from 'package:stats':
#> 
#>     filter
#> The following objects are masked from 'package:base':
#> 
#>     cbind, rbind
pool(list(sem("mpg ~ cyl", data = mtcars[1:10,]), sem("mpg ~ cyl", data = mtcars[1:20,])))
#> Error in object$coefficients: $ operator not defined for this S4 class

^{Created on 2024-01-14 with reprex v2.0.2}

Expected behavior That coef() works.

A solution would be to make get.dfcom a S3method, such that get.dfcom.default is the current function, and custom functions are implemented for lavaan and coxph (and in the future others?)