Registers form the core of the power of TV. These registers can be thought
of as a storage-area for either a string or a numerical value (essentially:
variables). Registers are called by a one-character long name. Valid names
are:
- all alphabetical characters (A - Z, no distinction between upper-
  and lower-case);
- all numerical characters (0 - 9);
- all shifted numerical characters on a USA keyboard (!, @, #, $, %, ^,
  &, *, (, ))
- The + character.
- The underscore character (_):
  This is a special type of register, containing the string TV currently
  regards as search-string. This register cannot be used for storing a
  number, but when retrieving a number from it, that number will be the
  size of the search-string.
- The characters representing F01 - F20 These are of a special type of
  register, containing the strings TV currently regards as the functions
  for the corresponding function keys. These registers cannot be used
  numerically.
- The character representing HELP. This is very similar to the
  function keys.

A register with textual content can be used to retrieve a number from;
that number will be the size of the string contained in the register.

All registers have a stack associated with them, allowing for enormous
flexibility in macro=-programming (including use as arrays).

The current register is also known as the buffer.