Palindrome for Addition

Context-free grammar for decimal point

S
=> aSa
=> aaSaa
=> aa+aEaaa
=> aa+aaEaaaa
=> aa+aa=aaaa
Accept

function delta(c, p) { // Unary addition
    if (c=='a' && p=='S') return "aSa";
    if (c=='+' && p=='S') return "+aEa";
    if (c=='a' && p=='E') return "aEa";
    if (c=='=' && p=='E') return "=";
    return '';  //default -- no transition
}
function generate(w, init='S') {
    //w: input String
    //init: start symbol
    //g: generated String
    let txt = init, g = init
    for (let i=0; i<w.length; i++) {
        let c = w[i], p = g[i]
        if (c == p) continue
        let d = delta(c, p)
        if (d == '') {
          input.selectionStart = i
          input.selectionEnd = i+1
          break
        }
        g = g.replace(p, d)
        txt += "\n=> "+g
    }
    return txt+'\n'+(w==g ? "Accept" : "Reject")
}