Make text lowercase

Um, I bet this is really simple (I’m as far as “beep”) but would someone tell me how to un-capitalize (make lower-case) a block of text. I though this should be quite simple using AppleScript.

I use this handler which can convert to upper or lower case.

set foo to "A Block Of Text to CONVERT"
set bar to my change_case(foo, "lower")

to change_case(txt, case_) -- case_ = "upper" or "lower"
	if case_ is "upper" then
		return do shell script "echo " & (quoted form of txt) & " | tr "[:lower:]" "[:upper:]""
	else
		if case_ is "lower" then
			return do shell script "echo " & (quoted form of txt) & " | tr "[:upper:]" "[:lower:]""
		end if
	end if
end change_case

Thank you!