Some yield in generator doesn't be mangled

[IsvaraWoo]

I use esmangle in NodeJS to compress following js:

function f(){
    return function *(next){
        var prev = next;
        prev = f.call(this, prev);
        yield *prev;
    };
}

I get the result as:

function f(){return function*(b){var a=b;a=f.call(this,a),yield*prev}}

The (last one) prev in source doesn't be mangled.

Compressor code in NodeJS:

var esprima = require('esprima');
var esmangle = require('esmangle');
var escodegen = require('escodegen');
var ast = esprima.parse(code);
// Get optimized AST
var optimized = esmangle.optimize(ast, null);
// gets mangled AST
var result = esmangle.mangle(optimized);
console.log(escodegen.generate(result, {
    format: {
        renumber: true,
        hexadecimal: true,
        escapeless: true,
        compact: true,
        semicolons: false,
        parentheses: false
    }
}));  // dump AST