Webster's English Dictionary

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obligatory

oblig.a.to.ry \*-'blig-*-.to-r-e-, a:-, -.to.r-; 'a:b-li-g*-\ aj 1: binding 
   in law or conscience  2: relating to or enforcing an obligation {a writ ~}  
   3: REQUIRED, MANDATORY  4: OBLIGATE