Create an html form pointing to a block: up to 3 arguments. 
 
- Argument 1: the name of the cited block. This can be a block of another
document, in this case this argument must be under the form ser/bl,
where ser is the serial number of the other document, and bl
the name of the block.
 If this argument is empty or equal to '.', it means the current block.
 
- Argument 2 (optional): the position to go (anchor) in the cited block.
 
- Argument 3: content of the form, in its usual html format.

The content of the form may contain up to 20 parameters under the names
of parm1, parm2, ..., parm20.
All other kinds of input parameters should be avoided.

For example, the following form allows the input of an arbitrary numerical expression. This expression is then evaluated in the document.

\form{.}{expform}{
Enter your expression:

}
\def{real value=\parm1}The expression is evaluated to: \value.

The source of the above:

\\form{.}{expform}{
Enter your expression:
<input size="30" name="parm1" value="\\parm1"/>
<input type="hidden" value="OK"/>
}
\\def{real value=\\parm1}The expression is evaluated to: \\value.

