parse(context, start) {
      this.context = context;
      const {
        src
      } = context;
      let offset = this.parseBlockHeader(start);
      offset = _Node.default.endOfWhiteSpace(src, offset);
      offset = this.parseComment(offset);
      offset = this.parseBlockValue(offset);
      return offset;
    }
Parses a block value from the source

Accepted forms are:
```
BS
block
lines

BS #comment
block
lines
```
where the block style BS matches the regexp `[|>][-+1-9]*` and block lines
are empty or have an indent level greater than `indent`.

@param {ParseContext} context
@param {number} start - Index of first character
@returns {number} - Index of the character after this block