>>-pos(needle-+--------+-)-------------------------------------><
              +-,start-+

Returns the position in the receiving buffer of another string, needle. (See also Section 5.4.15.18, “lastPos”.) 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). You can override this by specifying start (which must be a positive whole number), the point at which the search starts.

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