Undefined concepts used in Learning Manual

[John Sellers]

In reading through the Learning manual, all concepts seeming to be well 
introduced before using them in examples.  The clarity of these sections could 
not be better.

However in "section 2.5.1, Organizing pieces with variables"
it seems that several concepts are used in examples without having been 
previously defined.  Also new syntax is introduced without explanation.

These include

paper, variable, types, objects, new kinds of use of backslash, cm, 21.0 (e.g. 
numbers with decimal points), =, " " (e.g. quoted strings), line-width (e.g. 
control variables), etc.