[Groff] undocumented GNU eqn features

[Werner LEMBERG]

While studying eqn.y I've just discovered some features of GNU eqn which
appear to be undocumented in eqn.man.

. `undef' undefines a macro.

. `copy' is a synonym for `include'.

. `Alpha', `Beta', etc., is the same as `ALPHA', `BETA', etc.

. GNU eqn has two more predefined operators: `ldots' (three dots on
  the base line) and `dollar'.

. The keyword `space' sets the vertical spacing (in hundredths of em),
  normally before and after an equation.  It provides an interface to
  groff's \x escape but with opposite sign: positive values change the
  space before the equation line, negative values change the space
  after the equation line.

    .EQ
    space 50
    a = b + c
    space -300
    .EN

. The keywords `lcol' and friends can be immediately followed by a
  number which gives increases the vertical spacing of rows in
  hundredths of ems:

    .EQ
    matrix {
      ccol 100 { a above b above c }
      ccol     { d above e above f }
    }
   .EN

  Since this vertical spacing is realized with \x also, it interferes
  with the `space' command described above.

. The same is true for `lpile' and friends.


    Werner