+=(_:_:)

Positional arguments are concatenated:

var arguments: StatementArguments = [1]
arguments += [2, 3]

// Prints [1, 2, 3]
print(arguments)

Named arguments are inserted:

var arguments: StatementArguments = ["foo": 1]
arguments += ["bar": 2]

// Prints ["foo": 1, "bar": 2]
print(arguments)

If the arguments on the right-hand side has named parameters that are already defined on the left, a fatal error is raised:

var arguments: StatementArguments = ["foo": 1]

// fatal error: already defined statement argument: foo
arguments += ["foo": 2]

This fatal error can be avoided with the &+ operator, or the append(contentsOf:) method.

You can mix named and positional arguments (see the documentation of the StatementArguments type for more information about mixed arguments):

var arguments: StatementArguments = ["foo": 1]
arguments.append(contentsOf: [2, 3])

// Prints ["foo": 1, 2, 3]
print(arguments)