Simplification of Units
-----------------------

The simplifications described in this section are applied by the `u s'
(`calc-simplify-units') command.  These are in addition to the regular
`a s' (but not `a e') simplifications described earlier.  *Note Basic
Operations on Units::.

The variable `UnitSimpRules' contains rewrites to be applied by the `u
s' command.  These are applied in addition to `EvalRules' and
`AlgSimpRules'.

Scalar mode is automatically put into effect when simplifying units.
See Matrix Mode.

Sums `a + b' involving units are simplified by extracting the units of
`a' as if by the `u x' command (call the result `u_a'), then
simplifying the expression `b / u_a' using `u b' and `u s'.  If the
result has units then the sum is inconsistent and is left alone.
Otherwise, it is rewritten in terms of the units `u_a'.

If units auto-ranging mode is enabled, products or quotients in which
the first argument is a number which is out of range for the leading
unit are modified accordingly.

When cancelling and combining units in products and quotients, Calc
accounts for unit names that differ only in the prefix letter.  For
example, `2 km m' is simplified to `2000 m^2'.  However, compatible
but different units like `ft' and `in' are not combined in this way.

Quotients `a / b' are simplified in three additional ways.  First, if
`b' is a number or a product beginning with a number, Calc computes
the reciprocal of this number and moves it to the numerator.

Second, for each pair of unit names from the numerator and denominator
of a quotient, if the units are compatible (e.g., they are both units
of area) then they are replaced by the ratio between those units.  For
example, in `3 s in N / kg cm' the units `in / cm' will be replaced by
`2.54'.

Third, if the units in the quotient exactly cancel out, so that a `u
b' command on the quotient would produce a dimensionless number for an
answer, then the quotient simplifies to that number.

For powers and square roots, the "unsafe" simplifications `(a b)^c' to
`a^c b^c', `(a/b)^c' to `a^c / b^c', and `(a^b)^c' to `a^(b c)' are
done if the powers are real numbers.  (These are safe in the context
of units because all numbers involved can reasonably be assumed to be
real.)

Also, if a unit name is raised to a fractional power, and the base
units in that unit name all occur to powers which are a multiple of
the denominator of the power, then the unit name is expanded out into
its base units, which can then be simplified according to the previous
paragraph.  For example, `acre^1.5' is simplified by noting that `1.5
= 3:2', that `acre' is defined in terms of `m^2', and that the 2 in
the power of `m' is a multiple of 2 in `3:2'.  Thus, `acre^1.5' is
replaced by approximately `(4046 m^2)^1.5', which is then changed to
`4046^1.5 (m^2)^1.5', then to `257440 m^3'.

The functions `float', `frac', `clean', `abs', as well as `floor' and
the other integer truncation functions, applied to unit names or
products or quotients involving units, are simplified.  For example,
`round(1.6 in)' is changed to `round(1.6) round(in)'; the lefthand
term evaluates to 2, and the righthand term simplifies to `in'.

The functions `sin', `cos', and `tan' with arguments that have angular
units like `rad' or `arcmin' are simplified by converting to base
units (radians), then evaluating with the angular mode temporarily set
to radians.