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This version of hoc is slightly modified from the one
found in The Unix Programming Environment, by Brian Kernighan
and Rob Pike (Addison-Wesley, 1984).

Changes since the book version:

1.  The variable "_" contains the result of the last
top-level expression evaluation, so you can re-use
intermediate values without re-typing them.

2.  for statement added, like the one in C,
except that components may not be empty.

3.  ++ and -- added, but don't work with $i.

4.  Interrupts are caught; use cntrl-D to quit.

5.  % operator added.

6.  +=, -=, etc., added.

7.  \ at end of line eats newline.

8.  results are 15 digits but variable PREC can be
used to override this.

9.  tabs on output are no longer.

10. asin, acos, sinh, cosh, tanh, erf, erfc