Boolean option of whether or not we wish to use
the substitute command when implementing this
function. Note that this substitutes the argument of the
function. If substitute = FALSE, then the function will
instead treat the arguments as variables, and substitute in
their values.
command
character, the name of the function defining the
vector or list, e.g. "c", "list", "l". This let's the function
determine how many characters in front to remove.