swan
============================================================================
The swan JavaScript API include a `parse` function and a `createContext`
function to parse and evaluate expressions. 
  
swan.parse - function
----------------------------------------------------------------------------
Parses a swan expression and returns a function that maps a context to an
expression value.

```js
evaluate = swan.parse(expression);
value = await evaluate(context);
```

Where:
- `espression` is a string containing any valid swan expression
- `context` is a valid swan expression context
- `value` is the value that expression result has in the given context
  
swan.createContext - function
----------------------------------------------------------------------------
Creates a valid expression context.

```js
context = swan.createContext(...namespaces)
```

Where:
- `namespaces` is a list of objects `ns1, ns2, ns3, ...` that will be merged
  to the swan builtin namespace
- `context` is an object containing all the swan builtins, plus
  all the properties of the passed namespaces, added in order.