## Conversion of Units of Measurement

This program converts between different units
of measurement, or combinations of units.
A unit expression is:

- a unit name, such as
`meter`

.
Common abbreviations such as `ft`

or `mph`

are allowed.
- a number such as
`2.5`

or `kilo`

- a product of unit expressions,
`(* unit`_{1} ... unit_{n})

- a quotient of two unit expressions,
`(/ unit`_{1} unit_{2})

For example, a speed of *60 miles per hour* could be represented as
`(* 60 mph)`

or

`(* 60 (/ mile hour))`

.

For temperature conversions, the source must be a product such as
`(* 50 F)`

and the goal must be a temperature unit such as
`C`

.

## Instructions: