Because of the environment slot, simList objects don't correctly
memoise a simList.
This method for simList converts the object to a simList_ first.
# S3 method for class 'simList'
makeMemoisable(x)
# S3 method for class 'simList_'
unmakeMemoisable(x)A simList_ object or a simList, in the case of unmakeMemoisable.