+Time_description::set_time_signature (int l, int o)
+{
+ assert (o);
+ one_beat_ = Rational (1)/Moment (o);
+ whole_per_measure_ = Moment (l) * one_beat_;
+}
+
+bool
+Time_description::allow_time_signature_change_b()
+{
+ return!(whole_in_measure_);
+}
+
+/**
+ retrieve error messages.
+ @return
+ error messages if not possible, "" if possible
+ */
+String
+Time_description::try_set_partial_str (Moment p) const