(Corresponds to Section 4.8 of ANSI X3.9-1978 FORTRAN 77.)
A character constant may be delimited by a pair of double quotes
(") instead of apostrophes.
In this case, an apostrophe within the constant represents
a single apostrophe, while a double quote is represented in
the source text of the constant by two consecutive double
quotes with no intervening spaces.
A character constant may be empty (have a length of zero).
A character constant may include a substring specification,
The value of such a constant is the value of the substring--for
example, the value of 'hello'(3:5) is the same
as the value of 'llo'.