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Onus is an obligation, or a favor that needs to be repaid.

For Martians, it's especially important to repay onus in full. Croach the Tracker is very good at calculating how much onus is required.

Rules of Martian Onus

The rules for the tabulation of onus can be found in The Collected Scrolls of G'loot Praktaw, which are first referenced in La Venganza de Los Bandititos.

Per the scrolls, only the person under onus can place more onus upon themself, and onus cannot be placed by the person to whom the bearer is under onus (probably in order to keep extending the onus indefinitely).  

"Only the primary onus-bearer may transfer onus to person or persons under bearance or withholding completion thereof pursuant to primary onus."

Also, only the bearer can declare the onus complete, as learned in The Return of Croach the Tracker and the Introduction of Two New Important Characters!

Simply put, Sparks cannot place Croach under more onus to him, and Sparks cannot declare the onus complete.  Croach, however, as the primary onus-bearer, can place Sparks under onus to him.

Intention is measured against outcome in distribution of onus for those "of the tribe" only.