Masking a Layer how?

Hi everyone
I’d like to create a clip that contains a “writing effect”: first to be written is “a” and then “b” one after
the other.
When “a” shows up, only “a” is displayed. When “b” shows up, “ab” is displayed.
Using “text tool” in a layer called “text” i write “ab” (font: “_sans”).
Then i add a layer, calling it “mask”. In frame 1 i create a rectangle, (whereby using “rectangle”
tool) of width equals the width of the letter “a” of layer “text”. This rectangle is ungrouped !
I add a key frame (f6) to frame 10, i delete the rectangle and create a new rectangle of the width of
both “a” and “b” together from “text” layer. This rectangle is ungrouped too !
In frame 1 i select “Shape” from “tween” combo box.
From right click menu on “mask” layer i select "mask.
When i run that “fla” file the letter “a” appears first and then “b” on its’ side as desired.
I select: “Publish” from “file” menu and get a new “swf” file.
I run that swf and see nothing !!!
Any idea what went wrong ?
Thanks

Hi EastCoast,
Thanks
I tried “Arial” font and the same thing happened. “swf” showed nothing, “fla” showd gradually exposed letters.
Thanks

The textfield needs to use an embedded font, not a system font (which doesn’t get embedded)