Function language_to_sexp 

fn language_to_sexp(lang: &Language) -> Sexp

Convert a Language (call) to a LANGSXP pairlist.

Expr::Call { func, args } becomes Rf_lcons(func_sexp, Rf_cons(arg1, Rf_cons(arg2, ...))). Other expressions are stashed as sentinel LANGSXPs for Rf_eval round-trips.