Convert from pace [Roman] to X unit

Unit Definition (pace [Roman])
A pace (or double-pace) is a measure of distance used by Ancient Rome. It is the measure of a full stride from the position of the heel when it is raised from the ground to the point he heel is set down again at the end of the step. In Rome this was standardized as about five Roman feet or 58.1 English inches.

Unit Definition (X unit)
The X-unit (symbol xu) is a unit of length formerly used to measure the wavelength of X-rays and gamma rays. The X-unit is approximately 1.0021×10-13 metre. Defined by the Swedish physicist Karl Manne Georg Siegbahn (1886-1978) in 1925, the X-unit could not at that time be measured directly; the definition was instead made in terms of the spacing between planes of the calcite crystals used in the measuring apparatus.