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committed e4d5ea5

Some doc polishing.

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` N-Ary Addition and Multiplication`
` .................................`
` `
`+Addition and multiplication are n-ary, they can have an arbitrary number of `
`+arguments. ``1 + a + 2 + 3`` and ``1 * a * 2 * 3`` are respectively `
`+expressed as::`
`+`
`+    Add(List(Num(1.0), Sym("a"), Num(2.0), Num(3.0)))`
`+    Mul(List(Num(1.0), Sym("a"), Num(2.0), Num(3.0)))`
`+    `
` There are no nodes for subtraction or division. Subtraction is represented `
` as multiplication with ``-1``: (``-a = -1 * a``). Division is expressed as a `
`-power of ``-1``: (``1/a = a~^(-1)``). Addition and multiplication are also `
`-*n-ary*, they take an arbitrary number of arguments [#maxima]_. `
`+power of ``-1``: (``1/a = a~^(-1)``). [#maxima]_. `
` `
` As there are no subtraction or division operators, ``a-x`` and ``a/x`` are `
` respectively expressed as::`
`     Add(List(Sym("a"), Mul(List(Num(-1.0), Sym("x")))))`
`     Mul(List(Sym("a"), Pow(Sym("x"), Num(-1.0))))`
` `
`-Addition and multiplication are n-ary, they can have an arbitrary number of `
`-arguments. ``1 + a + 2 + 3`` and ``1 * a * 2 * 3`` are respectively `
`-expressed as::`
`-`
`-    Add(List(Num(1.0), Sym("a"), Num(2.0), Num(3.0)))`
`-    Mul(List(Num(1.0), Sym("a"), Num(2.0), Num(3.0)))`
`-    `
` Let Expressions`
` ................`
` `
`         let (a:=f, a\$x:=diff(f, x)) in diff(g, x)`
` `
` `
`-.. [#maxima] This idea was taken from the computer algebra program *Maxima*, it is intended `
`-       to simplify the algorithms.`
`+.. [#maxima] The ideas for n-ary operators (additon, multiplication), and `
`+   for the ommission of subtraction and division nodes, were taken from the `
`+   computer algebra program *Maxima*. It is intended to simplify the `
`+   algorithms.`
` `