e field to the shapefile (that must be newly created by
<a href="#label-9">ShapeFile::new</a>).
Argument <var>field_name</var> is a String for field name;
<var>field_type</var> specifies <a href="#label-87">field type</a>;
<var>width</var> is nonnegative Integer for field width in characters; and
<var>decimal</var> is nonnegative Integer for decimal points.
Only Float field requires <var>decimal</var>; otherwise it can be missing.</p>
<pre># example
fp.add_field 'name', :String, 32
fp.add_field 'height', "F