public void consume(char c) {
        if (s.charAt(p++)!=c)
            throw new IllegalArgumentException("Expected '" + c + "' " + at(p -1));
    }
Consumes the character at this position.
<br>Precondition: The character at this position is c.
<br>Postcondition: The position is increased by 1

@param c the expected character at this
@throws IllegalArgumentException if the character at this position is not c
vespajlib/src/main/java/com/yahoo/text/PositionedString.java