I was reading this often quoted article by Linda Wetzel (1993) where she discusses the 'occurence' of expressions in others and Quine's issues with the idea, she describes an expression as a sequence of symbols. If an expression can be a sequence of other expressions as well as a sequence of letters, how can both of these be true, defining a sentence as two separate things, mathematically speaking I would not define a sequence of letters as being also a sequence of words (a sequence with a sequence of letters at each position).
Perhaps I am misunderstanding here, but is she drawing a distinction between occurences in the string and occurences in the sentence as a type? I don't see how a sequence of letters and a sequence of words can be the same object as the function is different.
A function that takes a natural number to a letter, is different to a function that takes a natural number to a sequence of letters.
If 'expressions' are both strings of symbols how can one 'occur' within another? I find it a contradiction that an expression can occur within the other with the other retaining the same 'occurences' of symbols as it had before.
How can an 'expression' be a sequence of letters and a sequence of other sequences?