Q: Logstash Mutate I'm setting up ELK stack, and the following settings 
mutate {
  id => "adding_fields"
  add_field => {
    "host" => "foo-%{[beat][version]} baz"
    "custom" => "Hello, 1+%{host}+2"
  }
}

mutate {
  remove_field => [ "host" ]
}

ends up with to the following field content in the custom field: 
Hello, 1-name,ip-192-168-92-212,foo-6.4.2 baz-2.
How did this happen?
I exptexted only: Hello, 1+foo-6.4.2 baz+2
Where do the beginning of the field coming from?
The beat field contains the following data:
beat.name          |  beat.hostname      |  beat.version
ip-192-168-92-212  |  ip-192-168-92-212  |  6.4.2


A: I renamed the host field to myHost then the custom field became what I expected. 
It looks the host variable is already existing - but as a hidden field or something - but it is not sent to the ES.
