diff --git a/pmc/vector.cc b/pmc/vector.cc
--- a/pmc/vector.cc
+++ b/pmc/vector.cc
@@ -8,6 +8,8 @@
 
 #include <cstdio>
 
+using namespace algebra;
+
 Vector::Vector (int xx, int yy)
 {
     name = "Vector";
@@ -37,12 +39,12 @@
     return y;
 }
 
-Vector Vector::operator+ (Vector& a)
+Vector Vector::operator+ (Vector & a)
 {
     return Vector (x + a.X (), y + a.Y ());
 }
 
-Vector Vector::operator- (Vector& a)
+Vector Vector::operator- (Vector & a)
 {
     return Vector (x - a.X (), y - a.Y ());
 }
@@ -57,7 +59,7 @@
     fprintf (stderr, "(%.3d, %.3d)", x, y);
 }
 
-double Vector::angle (Vector& v)
+double Vector::angle (Vector & v)
 {
     if (mode == DEG)
         return ((180.0 / M_PI) * std::acos ((v.X () * X () + v.Y () * Y ()) / (abs () * v.abs ())));