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+Displays a message to the user and returns a value indicating the button that was clicked.
+
+    FUNCTION MessageBox& (Message$, Title$, Setup AS LONG)
+
+Not specifying Title$ will have the message box inherit the main form's title/caption.
+
+Setup is a combination (button + icon) of:
+* Buttons, which can be `MsgBox_OkOnly, MsgBox_OkCancel, MsgBox_AbortRetryIgnore, MsgBox_YesNoCancel, MsgBox_YesNo, MsgBox_RetryCancel, MsgBox_CancelTryagainContinue, MsgBox_CancelTryagainContinue`.
+* Icons, which can be `MsgBox_Critical, MsgBox_Question, MsgBox_Exclamation, MsgBox_Information`
+
+The possible return values are: `MsgBox_Ok, MsgBox_Cancel, MsgBox_Abort, MsgBox_Retry, MsgBox_Ignore, MsgBox_Yes, MsgBox_No, MsgBox_Tryagain, MsgBox_Continue`.
+
+All combinations of the above work as expected in Windows. For macOS and Linux, you only get OkOnly and YesNo for buttons.
+
+Sample usage:
+
+                Answer = MessageBox("Do you love QB64?", "", MsgBox_YesNo + MsgBox_Question)
+                IF Answer = MsgBox_No THEN
+                    Caption(Label1) = "I'll give you some more time to fall in love."
+                ELSE
+                    Caption(Label1) = "We do too!"
+                END IF