extern crate expression;
extern crate rand;
#[cfg(test)]
extern crate expression_num;

pub mod parametric;

use std::fmt::Debug;

/// Used to name symbols and variables.
pub trait Alphabet: Debug + PartialEq + Eq + Clone {}

impl Alphabet for &'static str {}
impl Alphabet for char {}
impl Alphabet for u8 {}
impl Alphabet for u16 {}
impl Alphabet for u32 {}
impl Alphabet for u64 {}
impl Alphabet for usize {}

/// An alphabet that distinguishes between terminal
/// and non-terminal symbols.
pub trait DualAlphabet: Alphabet {
    type Terminal;
    type NonTerminal: PartialOrd + Ord + Clone;

    /// If the character is a non-terminal, return Some(..). Otherwise return None.
    fn nonterminal(&self) -> Option<&Self::NonTerminal>;

    /// If the character is a terminal, return Some(..). Otherwise return None.
    fn terminal(&self) -> Option<&Self::Terminal>;
}

extern crate expression;
extern crate rand;
#[cfg(test)]
extern crate expression_num;

pub mod parametric;

use std::fmt::Debug;

/// Used to name symbols and variables.
pub trait Alphabet: Debug + PartialEq + Eq + PartialOrd + Ord + Clone {}

impl Alphabet for &'static str {}
impl Alphabet for char {}
impl Alphabet for u8 {}
impl Alphabet for u16 {}
impl Alphabet for u32 {}
impl Alphabet for u64 {}
impl Alphabet for usize {}

/// An alphabet that distinguishes between terminal
/// and non-terminal symbols.
pub trait DualAlphabet: Alphabet {
    type Terminal: Alphabet;
    type NonTerminal: Alphabet;

    /// If the character is a non-terminal, return Some(..). Otherwise return None.
    fn nonterminal(&self) -> Option<Self::NonTerminal>;

    /// If the character is a terminal, return Some(..). Otherwise return None.
    fn terminal(&self) -> Option<Self::Terminal>;

    /// Constructs non-terminal
    fn make_nonterminal(nt: Self::NonTerminal) -> Self;

    /// Constructs terminal
    fn make_terminal(t: Self::Terminal) -> Self;
}

--- 
+++ 
@@ -8,7 +8,7 @@
 use std::fmt::Debug;
 
 /// Used to name symbols and variables.
-pub trait Alphabet: Debug + PartialEq + Eq + PartialOrd + Ord + Clone {}
+pub trait Alphabet: Debug + PartialEq + Eq + Clone {}
 
 impl Alphabet for &'static str {}
 impl Alphabet for char {}
@@ -21,18 +21,12 @@
 /// An alphabet that distinguishes between terminal
 /// and non-terminal symbols.
 pub trait DualAlphabet: Alphabet {
-    type Terminal: Alphabet;
-    type NonTerminal: Alphabet;
+    type Terminal;
+    type NonTerminal: PartialOrd + Ord + Clone;
 
     /// If the character is a non-terminal, return Some(..). Otherwise return None.
-    fn nonterminal(&self) -> Option<Self::NonTerminal>;
+    fn nonterminal(&self) -> Option<&Self::NonTerminal>;
 
     /// If the character is a terminal, return Some(..). Otherwise return None.
-    fn terminal(&self) -> Option<Self::Terminal>;
-
-    /// Constructs non-terminal
-    fn make_nonterminal(nt: Self::NonTerminal) -> Self;
-
-    /// Constructs terminal
-    fn make_terminal(t: Self::Terminal) -> Self;
+    fn terminal(&self) -> Option<&Self::Terminal>;
 }
Simplify Alphabet and DualAlphabet definition and restrictions
