Use `g:sandwich#magicchar#f#patterns` or `b:sandwich_magicchar_f_patterns`.
Each of those are a list of patterns dictionary like:
let g:sandwich#magicchar#f#patterns = [
\ {
\ 'header' : '\<\h\k*',
\ 'bra' : '(',
\ 'ket' : ')',
\ 'footer' : '',
\ },
\ ]