composition

Compositions of configurations should be defined independent of the sources of those configurations.

Let us define a composition operator for configurations: `override`. **`override`** takes two configurations, `top` and `bottom`, and returns a new configuration with a union of every variable in the two configurations with the values from `top` taking precedence. In general, clients will start with a configuration of defaults and apply a sequence of overrides.

`override` may optionally throw, warn, or ignore overrides that do not exist in `bottom`. Ignore is a good option for migrating configurations across client versions. If a client updates from version 1 to 2 and drops configuration variable "x", a good migration tool will silently drop overrides of "x" in old configurations. Warn and throw are good for clients of varying pedantry.

`override` may optionally type check overrides. Some clients may want to delay validation until use, to avoid validating unused variables. Some may want pedantic validation of all overrides.