Semantics of << and <<-

Hi,
I recently joined this list, so please forgive me if this question has
been asked recently. I am wondering about the precise semantics of <<
and <<-. "Programming Ruby"[1] and the pseudo-BNFs[2][3] say that you
can use a quoted string after <<. As I see it, this means I should be
able to able to use the #{expr} construct inside this string, like this:
print <<"#{2+2}"
foobar
#{4}

This is, however, not the way my installation of Ruby (1.8.1) works.
What I am wondering is, is this the expected behaviour and are both the
book and the pseudo-BNFs wrong, or is this some form of bug in the
interpreter?

Best regards,
Anders

[1] Programming Ruby, 2nd Edition, p. 321
[2] http://www.ruby-doc.org/docs/ruby-doc-
bundle/Manual/man-1.4/yacc.html
[3] http://www.ruby-lang.org/ja/man/?cmd=view;name=%B5%BF%BB%F7BNF%A4%CB
%A4%E8%A4%EBRuby%A4%CE%CA%B8%CB%A1



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