Returns the position in the receiving buffer of another string, needle. It returns 0 if needle is the null string or is not found or if start is greater than the length of the receiving buffer. By default, the search starts at the first character of the receiving buffer (that is, the value of start is 1), and continues to the end of the string. You can override this by specifying start, the point at which the search starts, and length, the bounding limit for the search. If specified, start must be a positive whole number and length must be a non-negative whole number.

See also method lastPos.

x1 = .mutablebuffer~new("Saturday")
x1~pos("day")       -->    6
x1 = .mutablebuffer~new("abc def ghi")
x1~pos("x")         -->    0
x1~pos(" ")         -->    4
x1~pos(" ",5)       -->    8