statement takes two arguments
. Each of these arguments needs to be something that yields a string
value (a sequence of letters). The value of the variable $my_text
is a string, specifically the selection you assigned it earlier. The letter a is however nothing that yields a value. What the macro language requires is that you tell it you want a string containing the letter a. You do this by enclosing it in quotation marks.
To review, the following are all valid macros: