private void noFieldShadowing(String name) {
        if (fields.containsKey(name)) {
            throw new IllegalArgumentException("Schema '" + getName() + "' already contains the fields '" + fields +
                                               "'. You are trying to add '" + name + "'. Shadowing is not supported");
        }
    }
Adds a document that can be mapped to this schema.

@param source a document that can be mapped to this schema.
@return Itself for chaining.
config-model/src/main/java/com/yahoo/vespa/documentmodel/SchemaDef.java