Numbers are one of two types:
    int
           no decimal point
           approx range plus or minus 2,000,000,000
    double
           with decimal point
           accurate to approx 14 digits
           range approx ten-to-the-power-of plus or minus 308
           may be written using "e" to stand for "times ten
               to the power of", e.g.
                    1.6e6 means 1600000.0
                    1.23e-4 means 0.000123

Expressions produce a value
            generally have no direct effect.
    consist of
         numeric constants
         operators
                    * /   high priority
                    + -   low priority
                    others to be seen
                        same priority goes left to right.
                        operator given two ints always has int result
                        two doubles gives double result
                        mixture: int converted to double first. 
         ( ) 
         functions   e.g.  sqrt(2.5)
              these ones take double inputs and give double results
                         sin, cos, tan, atan, asin, acos,
                              they use radians not degrees.
                         sqrt
                         fabs is absolute value
                         pow(a, b) is a to the power of b
                         exp(a) is e to the power of a
                         log(a) is natural (base e) logarithm of a
                         log10(a) is base 10 logarithm
                    there are many more
              this one take int inputs and gives int results
                         abs, absolute value
statements
   have a direct effect, generally don't produce results
   always end with a semicolon.
   e.g. print(6*3);
        make_window(400, 600-22);
   many more kinds, to be seen soon.

Names must begin with a letter, and may contain only letters, digits,
     and _underlines. C++ thinks that cat, Cat, and CAT are three
     completely different things.

Comments are completely ignored and have no effect. They can appear
   anywhere. The have two forms:

   //   after the symbol // everything up to the end of the line
        is ignored

   /*   after the symbol /* everything up to the symbol */ is
        ignored. This sort of comment can cover many many lines.

A program consists of a sequence of

  directives - begin with #   #include "library.h"

  definitions of functions.
     group of statements given a name

       void name ( )
       { a bunch of statements }

       other forms will be seen soon.

  definitions of constants, for example

     const int hundred = 100;

     const double pi = acos(-1.0);  // this is the most accurate pi.