int GiD_fWriteElementMat(GiD_FILE fd, int id, int nid[]);
int GiD_fWriteSphereMat(GiD_FILE fd, int id, int nid, double r, int mat);
int GiD_fWriteCircleMat(GiD_FILE fd, int id, int nid, double r, double nx, double ny, double nz, int mat);
Description: Similar to GiD_WriteElement, but the last aditional integer
on nid correspond to a material number.
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