diff --git a/src/lib.rs b/src/lib.rs
index 3745178..55c47fc 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,6 +1,8 @@
 use std::collections::{HashMap, HashSet};
 use std::env::args_os;
+use std::ops::RangeBounds;
 
+/// A command line argument parser.
 #[derive(Debug, Clone)]
 pub struct Parser {
     flags: Vec<Flag>,
@@ -12,17 +14,107 @@ pub struct Parser {
     short_set: HashSet<String>,
 }
 
+impl Parser {
+    pub fn new() -> Parser {
+        Parser {
+            flags: Vec::new(),
+            args: Vec::new(),
+            pos_args: Vec::new(),
+            long_set: HashSet::new(),
+            short_set: HashSet::new(),
+        }
+    }
+
+    /// Add a flag argument.
+    pub fn flag(&mut self, long: &str, short: Option<&str>, doc_string: &str) {
+        self.ensure_not_duplicate(long, short);
+
+        self.flags.push(Flag {
+            long: long.into(),
+            short: short.map(|s| s.into()),
+            doc_string: doc_string.into(),
+        });
+    }
+
+    /// Add a standard argument.
+    pub fn argument<R: RangeBounds<usize>>(
+        &mut self,
+        long: &str,
+        short: Option<&str>,
+        doc_string: &str,
+        value_name: &str,
+        acceptable_count: R,
+    ) {
+        self.ensure_not_duplicate(long, short);
+
+        use std::ops::Bound;
+        let count_start: Option<usize> = match acceptable_count.start_bound() {
+            Bound::Included(&n) => Some(n),
+            Bound::Excluded(&n) => Some(n + 1),
+            Bound::Unbounded => None,
+        };
+        let count_end: Option<usize> = match acceptable_count.end_bound() {
+            Bound::Included(&n) => Some(n + 1),
+            Bound::Excluded(&n) => Some(n),
+            Bound::Unbounded => None,
+        };
+
+        self.args.push(Arg {
+            long: long.into(),
+            short: short.map(|s| s.into()),
+            acceptable_count: (count_start, count_end),
+            value_name: value_name.into(),
+            doc_string: doc_string.into(),
+        });
+    }
+
+    /// Add a positional argument.
+    ///
+    /// There can only be one positional argument with `count == None`,
+    /// which represents any left over positional arguments after those
+    /// with explicit counts have been parsed.
+    pub fn positional_argument(
+        &mut self,
+        doc_string: &str,
+        value_name: &str,
+        count: Option<usize>,
+    ) {
+        self.pos_args.push(PosArg {
+            count: count,
+            value_name: value_name.into(),
+            doc_string: doc_string.into(),
+        });
+    }
+
+    //----------------
+
+    fn ensure_not_duplicate(&mut self, long: &str, short: Option<&str>) {
+        if self.long_set.contains(long) {
+            panic!("Attempted to add duplicate long argument \"--{}\".", long);
+        }
+        self.long_set.insert(long.into());
+
+        if let Some(short) = short {
+            if self.short_set.contains(short) {
+                panic!("Attempted to add duplicate short argument \"-{}\".", short);
+            }
+            self.short_set.insert(short.into());
+        }
+    }
+}
+
+/// Parsed command line arguments.
 #[derive(Debug, Clone)]
 pub struct Arguments {
     // TODO: allow OsString argument values.
-    /// Positional arguments, in the order passed.
-    pub positional: Vec<String>,
-
-    /// The flags that were passed.
+    /// The flags that were passed, indexed by `long`.
     pub flags: HashSet<String>,
 
-    /// Non-positional arguments that were passed.
-    pub args: HashMap<String, String>,
+    /// Non-positional arguments that were passed, indexed by `long`.
+    pub args: HashMap<String, Vec<String>>,
+
+    /// Positional arguments, indexed by `value_name`.
+    pub positional: HashMap<String, Vec<String>>,
 }
 
 //-------------------------------------------------------------
@@ -30,7 +122,7 @@ pub struct Arguments {
 /// Flag spec.
 #[derive(Debug, Clone)]
 struct Flag {
-    long: Option<String>,
+    long: String,
     short: Option<String>,
 
     // For documentation.
@@ -40,9 +132,9 @@ struct Flag {
 /// Argument spec.
 #[derive(Debug, Clone)]
 struct Arg {
-    long: Option<String>,
+    long: String,
     short: Option<String>,
-    is_required: bool,
+    acceptable_count: (Option<usize>, Option<usize>),
 
     // For documentation.
     value_name: String,
@@ -52,7 +144,7 @@ struct Arg {
 /// Positional argument spec.
 #[derive(Debug, Clone)]
 struct PosArg {
-    is_required: bool,
+    count: Option<usize>,
 
     // For documentation.
     value_name: String,