How to SET the text in a text cast member:

member("textentry").text = "I like Trogdor"

Or

x = "I hate Trogdor"
member("textentry").text = x

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How to SET the text in a text cast member to be EMPTY:

member("textentry").text = ""

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How to GET the text out of a text cast member, and put the text
into a variable:

x = member("textentry").text 

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The Editable property of text cast members:

You can set text cast members to be editable, so that users can type in to them. 
You can then use scripting to get what users type, and put it into a variable for your use.

Remember, if you want to get info from users, you have to have a text cast member on the stage, 
and you have to set the editable to TRUE -- this way users can type into the text cast member! 

Setting the Editable:
When a text cast member is on the stage, you select it, and then you can press the editable button 
(looks like the cursor between an A and a B) to make the text cast member editable.

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