Adding expr() methods

This API mimics the API of smoek.  We'll see if we keep this, but it
seems to make sense.

Author: whart222

lib/coek/coek/api/constraint.cpp

@@ -29,6 +29,12 @@ Constraint& Constraint::operator=(const Constraint& expr)
     return *this;
 }
 
+Constraint& Constraint::expr(const Constraint& expr)
+{ repn = expr.repn; return *this; }
+
+Constraint Constraint::expr() const
+{ return *this; }
+
 bool Constraint::active() const { return repn->active; }
 
 void Constraint::activate() { repn->active = true; }

lib/coek/coek/api/constraint.hpp

@@ -35,13 +35,19 @@ class Constraint {
 
     Constraint& operator=(const Constraint& arg);
 
+    /** Set the constraint */
+    Constraint& expr(const Constraint& body);
+    /** \returns the constraint */
+    Constraint expr() const;
+
     /** \returns \c true if this is an inequality constraint */
     bool is_inequality() const;
     /** \returns \c true if this is an equality constraint */
     bool is_equality() const;
     /** \returns \c true if the constraint is feasible */
     bool is_feasible() const;
 
+
     /** \returns the expression for the constraint lower bound */
     Expression lower() const;
     /** \returns the expression for the constraint body */