I was thinking about whether there is a difference between the symbol 'x' and the expression/formula containing it.
For example we discuss 'x' the symbol but equally I can talk about 'x' the expression, if a wff is defined as a subset of sequences of symbols, then the formula 'x' is a sequence containing one position with which the symbol 'x' is occurs.
Is there a formal difference between these two? Mathematically a sequence is a function mapping elements to positions, mathematically this makes them different, however if I analyse a sequence 'x+y' I see that there is one occurence of both the symbol 'x' and sequence 'x'(sequence with only the symbol 'x' in it) however both correspond to the same position in the same sequence.