Create the parsing Tfxm.t. The parser is "best-effort" and stateless (i.e. a line containing "1000 42." will parsed successfully as a `variable_step_value (1000, 42.) even if no "variableStep" was line present before).

Create the transform that prints item values to strings.

Create a transform which converts `variable_step_value _ and `fixed_step_value _ values to `bed_graph_value _ values, using the current state. The `bed_graph_value _ items stay untouched and `comment _ values are ignored.