a. Competency. After Bartick identified two white men at the voir dire, the defendant moved for a competency examination pursuant to G. L. c. 123, Section 19 (1986 ed.). [Note 11] In these circumstances, the judge should have ordered a competency evaluation pursuant to G. L. c. 123, Section 19. "If the competency of a witness is placed in issue, `it is the duty of the judge to examine into the question of [the witness's] competency, and to reject [the witness] unless [the judge] is satisfied that [the witness] is competent.'" Commonwealth v. Gibbons, 378 Mass. 766 , 770 (1979), quoting Commonwealth v. Reagan, 175 Mass. 335 , 340 (1900). Testimony from an expert on Down's syndrome is not the equivalent of following the mandate of the statute that an impartial expert be assigned by the Department of Mental Health. [Note 12]