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+# once
+
+Only call a function once.
+
+## usage
+
+```javascript
+var once = require('once')
+
+function load (file, cb) {
+  cb = once(cb)
+  loader.load('file')
+  loader.once('load', cb)
+  loader.once('error', cb)
+}
+```
+
+Or add to the Function.prototype in a responsible way:
+
+```javascript
+// only has to be done once
+require('once').proto()
+
+function load (file, cb) {
+  cb = cb.once()
+  loader.load('file')
+  loader.once('load', cb)
+  loader.once('error', cb)
+}
+```
+
+Ironically, the prototype feature makes this module twice as
+complicated as necessary.
+
+To check whether you function has been called, use `fn.called`. Once the
+function is called for the first time the return value of the original
+function is saved in `fn.value` and subsequent calls will continue to
+return this value.
+
+```javascript
+var once = require('once')
+
+function load (cb) {
+  cb = once(cb)
+  var stream = createStream()
+  stream.once('data', cb)
+  stream.once('end', function () {
+    if (!cb.called) cb(new Error('not found'))
+  })
+}
+```
+
+## `once.strict(func)`
+
+Throw an error if the function is called twice.
+
+Some functions are expected to be called only once. Using `once` for them would
+potentially hide logical errors.
+
+In the example below, the `greet` function has to call the callback only once:
+
+```javascript
+function greet (name, cb) {
+  // return is missing from the if statement
+  // when no name is passed, the callback is called twice
+  if (!name) cb('Hello anonymous')
+  cb('Hello ' + name)
+}
+
+function log (msg) {
+  console.log(msg)
+}
+
+// this will print 'Hello anonymous' but the logical error will be missed
+greet(null, once(msg))
+
+// once.strict will print 'Hello anonymous' and throw an error when the callback will be called the second time
+greet(null, once.strict(msg))
+```