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A general description is here.
New experimental API for working with variable bindings in environments. The function ‘R_GetBindingType’ identifies the binding type without forcing a delayed or active binding. New bindings care created by ‘R_MakeDelayedBinding’, ‘R_MakeForcedBinding’ and ‘R_MakeMissingBinding’. Delayed and forced bindings can be inspected with ‘R_ForcedBindingExpression’, ‘R_DelayedBindingExpression’, and ‘R_DelayedBindingEnvironment’. Bindings for ‘...’ in function call environments can be examined with ‘R_DotsExist’, ‘R_DotsLength’, ‘R_DotsNames’, ‘R_DotsElt’, ‘R_GetDotType’, ‘R_DotDelayedExpression’, ‘R_DotDelayedEnvironment’, and ‘R_DotForcedExpression’. Contributed by Lionel Henry and Davis Vaughan in PR#18928.
The functions ‘Rf_findVar’ and ‘findVarInFrame’ are now marked as non-API, and uses in packages will now receive check ‘WARNING’s. See ‘Writing R Extensions’ for alternatives to use.