Substitutions in Language Objects
Substitute differs from
in so far as its first argument can be a variable that
contains an object of mode "language". In that case,
substitutions take place inside this object.
any object, unevaluated expression, or unevaluated language construct, such as a sequence of calls inside braces
a named list, environment, data frame or data set.
The function body is just
An object of storage mode "language" or "symbol".
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