foundryvtt-wh4-lang-fr-fr/reference_scripts/HrOBAXsEX073ReKl.js

let skill = `Entertain (Singing)`
let currentCareer = this.actor.system.currentCareer;
let existingSkill = this.actor.itemTypes.skill.find(i => i.name == skill);

if (!currentCareer) return


let inCurrentCareer = currentCareer.system.skills.includes(skill);
let perfectPitchAdded = this.actor.getFlag("wfrp4e", "perfectPitchAdded") || {};
if (existingSkill && inCurrentCareer && !perfectPitchAdded[existingSkill.name])
{
	existingSkill.system.advances.costModifier = -5;
}
else 
{
	perfectPitchAdded[skill] = true;
	currentCareer.system.skills.push(skill);
	setProperty(this.actor, "flags.wfrp4e.perfectPitchAdded", perfectPitchAdded)
}