API Blueprint `Mixin Type` "Include" in Data Structure was not included/parsed in the output

[akasection]

Hello!

I encountered a weird problem when converting a blueprint that has data structure with Include (which is mixin type) in the spec, but then it seems to be ignored in the component schema.

For example, given this apib:

# Test API

# Data Structures

## `Person` (object)

- first_name (string, required)
- last_name (string, required)

## `Member` (object)

- uid (string, required)
- Include `Person`

# Group Members

## Member Registrations [/accounts/register]

### Submit Registration [POST /accounts/register]

Register a user

- Request (application/json)
    - Attributes (Person)

- Response 201 (application/json)
    - Attributes (Member)

The API result seems to be okay, but the component schema "Member" does not refer to "Person" at all, so it's missing the first_name and last_name.

{
  "openapi": "3.0.3",
  "info": {
    "title": "Test API",
    "version": "1.0.0",
    "description": ""
  },
  "paths": {
    "/accounts/register": {
      "post": {
        "responses": {
          "201": {
            "description": "Created",
            "headers": {},
            "content": {
              "application/json": {
                "schema": {
                  "type": "object",
                  "properties": {
                    "uid": {
                      "type": "string"
                    },
                    "first_name": {
                      "type": "string"
                    },
                    "last_name": {
                      "type": "string"
                    }
                  },
                  "required": ["uid", "first_name", "last_name"]
                },
                "example": {
                  "uid": "",
                  "first_name": "",
                  "last_name": ""
                }
              }
            }
          }
        },
        "summary": "Submit Registration",
        "operationId": "Submit Registration",
        "description": "Register a user",
        "tags": ["Members"],
        "parameters": [],
        "requestBody": {
          "content": {
            "application/json": {
              "example": {
                "first_name": "",
                "last_name": ""
              },
              "schema": {
                "type": "object",
                "properties": {
                  "first_name": {
                    "type": "string"
                  },
                  "last_name": {
                    "type": "string"
                  }
                },
                "required": ["first_name", "last_name"]
              }
            }
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
      "Person": {
        "type": "object",
        "required": ["first_name", "last_name"],
        "properties": {
          "first_name": {
            "type": "string"
          },
          "last_name": {
            "type": "string"
          }
        }
      },
      "Member": {
        "type": "object",
        "required": ["uid"],
        "properties": {
          "uid": {
            "type": "string"
          }
        }
      }
    }
  },
  "tags": [
    {
      "name": "Members"
    }
  ]
}

Do I miss something or the converter simply unaware about mixins?