Reads the input source map and returns a symbol position in the input source (e.g., in a Sass file that was compiled to CSS before being passed to PostCSS). Optionally takes an end position, exclusive.

root.source.input.origin(1, 1) //=> { file: 'a.css', line: 3, column: 1 }
root.source.input.origin(1, 1, 1, 4)
//=> { file: 'a.css', line: 3, column: 1, endLine: 3, endColumn: 4 }