new_theory : string -> unit
STRUCTURE
Theory
SYNOPSIS
Creates a new theory segment.
DESCRIPTION
A theory consists of a hierarchy of named parts called ‘theory segments’. All theory segments have a ‘theory’ of the same name associated with them consisting of the theory segment itself together with the contents of all its ancestors. Each axiom, definition, specification and theorem belongs to a particular theory segment.

Calling new_theory thy creates a new, and empty, theory segment having name thy. The theory segment which was current before the call becomes a parent of the new theory segment. The new theory therefore consists of the current theory extended with the new theory segment. The new theory segment replaces its parent as the current theory segment. The parent segment is exported to disk.

In the interests of interactive usability, the behaviour of new_theory has some special cases. First, if new_theory thy is called in a situation where the current theory segment is already called thy, then this is interpreted as the user wanting to restart the current segment. In that case, the current segment is wiped clean (types and constants declared in it are removed from the signature, and all definitions, theorems and axioms are deleted) but is otherwise unchanged (it keeps the same parents, for example).

Second, if the current theory segment is empty and named "scratch", then new_theory thy creates a new theory segment the parents of which are the parents of "scratch". (This situation is almost never visible to users.)

FAILURE
A call new_theory thy fails if the name thy is unsuitable for use as a filename. In particular, it should be an alphanumeric identifier.

Failure also occurs if thy is the name of a currently loaded theory segment. In general, all theory names, whether loaded or not, should be distinct. Moreover, the names should be distinct even when case distinctions are ignored.

EXAMPLE
In the following, we follow a standard progression: start HOL up and declare a new theory segment. 
   - current_theory();
   > val it = "scratch" : string

   - parents "-";
   > val it = ["list", "option"] : string list

   - new_theory "foo";
   <<HOL message: Created theory "foo">>
   > val it = () : unit

   - parents "-";
   > val it = ["list", "option"] : string list
Next we make a definition, prove and store a theorem, and then change our mind about the name of the defined constant. Restarting the current theory keeps the static theory context fixed but clears the current segment so that we have a clean slate to work from. 
   - val def = new_definition("foo", Term `foo x = x + x`);
   > val def = |- !x. foo x = x + x : thm

   val thm = Q.store_thm("foo_thm", `foo x = 2 * x`,
                                    RW_TAC arith_ss [def]);
   > val thm = |- foo x = 2 * x : thm

   - new_theory "foo";
   <<HOL message: Restarting theory "foo">>
   > val it = () : unit

   val def = new_definition("twice", Term `twice x = x + x`);
   > val def = |- !x. twice x = x + x : thm

   - curr_defs();
   > val it = [("twice", |- !x. twice x = x + x)]
              : (string * thm) list

COMMENTS
The theory file in which the data of the new theory segment is ultimately stored will have name thyTheory in the directory in which export_theory is called.
USES
Modularizing large formalizations. By splitting a formalization effort into theory segments by use of new_theory, the work required if definitions, etc., need to be changed is minimized. Only the associated segment and its descendants need be redefined.
SEEALSO
current_theory, new_axiom, parents, new_binder, new_constant, new_definition, new_infix, new_specification, new_type, save_thm, export_theory, print_theory
HOL  Kananaskis-14