Naturally supported single variable in lambdas

According to the book:

The formal parameters defined by formals are bound to the actual
parameters as follows.

  *  If formals is a proper list of variables, e.g., (x y z), each
     variable is bound to the corresponding actual parameter. It is an
     error if too few or too many actual parameters are supplied.

  *  If formals is a single variable (not in a list), e.g., z, it is
     bound to a list of the actual parameters.

  *  If formals is an improper list of variables terminated by a
     variable, e.g., (x y . z), each variable but the last is bound to
     the corresponding actual parameter. The last variable is bound to
     a list of the remaining actual parameters. It is an error if too
     few actual parameters are supplied.

(http://www.scheme.com/tspl3/binding.html#./binding:h2)