rxToSE() and rxFromSE() remember the translations they have made, so
text that is translated again (as symbolic-derivative builds do, line after
line) is not re-parsed. The cache is cleared automatically when a new
translation session starts (rxS(), or rxToSE() with an envir), when
the derivative or user-function tables change, and when it grows past
getOption("rxode2.symengineCacheMB", 256) megabytes. Set
options(rxode2.symengineCache = FALSE) to turn it off.