Generating C# classes from .proto files on a Mac

proto-on-mac-example.sh

$ cat example.proto
message AMessage
{
    required int32 a=1;
    optional int32 b=2;
}

$ protoc example.proto --descriptor_set_out=example.pb
$ mono ProtoGen.exe example.pb
$ more Example.cs
// Generated by the protocol buffer compiler.  DO NOT EDIT!

using pb = global::Google.ProtocolBuffers;
using pbc = global::Google.ProtocolBuffers.Collections;
using pbd = global::Google.ProtocolBuffers.Descriptors;
using scg = global::System.Collections.Generic;
public static partial class Example {
...
...

Works fine

Where is the ProtoGen.exe?

thank you very much