Minor fixes

Based on William Rucklidge's review, including
a nasty bug in parameter block removal.

Change-Id: I3a692e589f600ff560ecae9fa85bb0b76063d403

internal/ceres/preconditioner.h

diff --git a/internal/ceres/preconditioner.h b/internal/ceres/preconditioner.h
index 77280c3..5bb077e 100644
--- a/internal/ceres/preconditioner.h
+++ b/internal/ceres/preconditioner.h
@@ -69,11 +69,11 @@
     //
     // For example if elimination_groups is a vector of size k, then
     // the linear solver is informed that it should eliminate the
-    // parameter blocks 0 - elimination_groups[0] - 1 first, and then
-    // elimination_groups[0] - elimination_groups[1] and so on. Within
-    // each elimination group, the linear solver is free to choose how
-    // the parameter blocks are ordered. Different linear solvers have
-    // differing requirements on elimination_groups.
+    // parameter blocks 0 ... elimination_groups[0] - 1 first, and
+    // then elimination_groups[0] ... elimination_groups[1] and so
+    // on. Within each elimination group, the linear solver is free to
+    // choose how the parameter blocks are ordered. Different linear
+    // solvers have differing requirements on elimination_groups.
     //
     // The most common use is for Schur type solvers, where there
     // should be at least two elimination groups and the first
@@ -130,7 +130,7 @@
 class SparseMatrixPreconditionerWrapper : public Preconditioner {
  public:
   // Wrapper does NOT take ownership of the matrix pointer.
-  SparseMatrixPreconditionerWrapper(const SparseMatrix* matrix);
+  explicit SparseMatrixPreconditionerWrapper(const SparseMatrix* matrix);
   virtual ~SparseMatrixPreconditionerWrapper();
 
   // Preconditioner interface