+.pk: Add a `pkproto` object to a `pk` object
Description
Add a `pkproto` object to a `pk` object
Usage
# S3 method for pk
+(e1, e2)
Value
The `pk` object, modified by adding the `pkproto` object
Arguments
- e1
A `pk` pbject
- e2
A `pkproto` object
Details
Note that `e1 + e2` is equivalent to
```
`+`(e1, e2)
```