Position & Inventory Functions
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.. py:type:: position

   A position is a single amount held at cost.

   Example: `10 HOOL {100.30 USD}`

.. py:type:: inventory

   A collection of multiple positions is an inventory. 

Positions can be :func:`summed<func.sum>` up, resulting in 
:type:`inventories<inventory>`.

Functions to inspect or modify positions and inventories:

* :func:`~fun.abs` - Absolute value.

* :func:`~fun.convert` - Coerce an amount to a particular currency.

* :func:`~fun.cost` - Get the cost of a position.

* :func:`~fun.empty` - Determine whether the inventiry is empty.

* :func:`~fun.filter_currency` - Filter an inventory to just the specified currency.

* :func:`~fun.getprice` - Fetch a price.

* :func:`~fun.neg` - Negative value.

* :func:`~fun.possign` - Correct sign of an Amount based on the usual balance of associated account.

* :func:`~fun.units` - Get the number of units.

* :func:`~fun.value` - Coerce an inventory to its market value.