+/** \fn stridentcnt_P(char* a, PGM_P b)
+ * \brief counts the number of identic chars
+ *
+ * This function compares the supplyed strings and returns the index of the
+ * first char where the strings differ.
+ * \param a pointer to string in RAM
+ * \param b pointer to string in Flash
+ * \return index of the first char where \c a and \c b differ
+ */