manual split up into multiple files; documented the new concurrency system

doc/manual/definitions.txt

@@ -0,0 +1,49 @@
+
+Definitions
+===========
+
+A Nim program specifies a computation that acts on a memory consisting of
+components called `locations`:idx:. A variable is basically a name for a
+location. Each variable and location is of a certain `type`:idx:. The
+variable's type is called `static type`:idx:, the location's type is called
+`dynamic type`:idx:. If the static type is not the same as the dynamic type,
+it is a super-type or subtype of the dynamic type.
+
+An `identifier`:idx: is a symbol declared as a name for a variable, type,
+procedure, etc. The region of the program over which a declaration applies is
+called the `scope`:idx: of the declaration. Scopes can be nested. The meaning
+of an identifier is determined by the smallest enclosing scope in which the
+identifier is declared unless overloading resolution rules suggest otherwise.
+
+An expression specifies a computation that produces a value or location.
+Expressions that produce locations are called `l-values`:idx:. An l-value
+can denote either a location or the value the location contains, depending on
+the context. Expressions whose values can be determined statically are called
+`constant expressions`:idx:; they are never l-values.
+
+A `static error`:idx: is an error that the implementation detects before
+program execution. Unless explicitly classified, an error is a static error.
+
+A `checked runtime error`:idx: is an error that the implementation detects
+and reports at runtime. The method for reporting such errors is via *raising
+exceptions* or *dying with a fatal error*. However, the implementation 
+provides a means to disable these runtime checks. See the section pragmas_
+for details. 
+
+Wether a checked runtime error results in an exception or in a fatal error at
+runtime is implementation specific. Thus the following program is always
+invalid:
+
+.. code-block:: nim
+  var a: array[0..1, char]
+  let i = 5
+  try:
+    a[i] = 'N'
+  except EInvalidIndex:
+    echo "invalid index"
+
+An `unchecked runtime error`:idx: is an error that is not guaranteed to be
+detected, and can cause the subsequent behavior of the computation to
+be arbitrary. Unchecked runtime errors cannot occur if only `safe`:idx:
+language features are used.
+