class input_string :?pos:int -> ?len:int -> string ->in_obj_channel
The method pos_in reflects the real position in the string, i.e.
a character read at position k can be found at s.[k] in the string
s.
pos : The data of the channel begins at this position of the string.
Default: 0len : The data of the channel consists of this number of bytes.
Default: until the end of the string