A signal of danger or a problem can be referred to as a red flag, a usage that originated in the 18th century. An infamous example of use of a red flag in warfare is Mexican General Santa Anna's use of the symbol to let his Texian opposition in the Alamo know that he intended to spare none of the defenders (on which he followed through). The term "red flag" is used, e.g., during screening of communications, and refers to specific words or phrases encountered that might indicate relevance to the case. For example, email spam filters make use of such "red flags".