Syntax tree nodes for language signature items, of type
Ast.sig_item. The basic nodes are:
<:sig_item< declare $list:sil$ end >>: declare.
<:sig_item< exception $s$ of $list:tl$ >>:
exception declaration.
<:sig_item< external $s$ : $t$ = $list:sl$ >>:
declaration of external.
<:sig_item< module $s$ : $mt$ >>: module declaration.
<:sig_item< module type $s$ = $mt$ >>: module type
declaration.
<:sig_item< open $sl$ >>: open.
<:sig_item< type $list:ssltl$ >>: type declaration.
<:sig_item< value $s$ : $t$ >>: variable declaration.