Now match element will default to identity matrix if no betas are set. (

#1138)

bmad/code/lat_sanity_check.f90

@@ -583,18 +583,21 @@ subroutine lat_sanity_check (lat, err_flag)
       match_std   = (is_true(ele%value(recalc$)) .and. nint(ele%value(matrix$)) == standard$)
       match_orbit = (is_true(ele%value(recalc$)) .and. nint(ele%value(kick0$)) == match_orbit$)
 
-      if ((match_std .or. match_twiss) .and. (ele%value(beta_a1$) <= 0 .or. ele%value(beta_b1$) <= 0)) then
-        call out_io (s_fatal$, r_name, &
-                      'ELEMENT: ' // ele_full_name(ele, '@N (&#)'), &
-                      'WHICH IS A MATCH ELEMENT HAS A BETA_A1 OR BETA_B1 THAT IS NOT POSITIVE.')
-        err_flag = .true.
-      endif
+      if (.not. (match_std .and. ele%value(beta_a1$) == 0 .and. ele%value(beta_b1$) == 0 .and. &
+                                 ele%value(beta_a0$) == 0 .and. ele%value(beta_b0$) == 0)) then
+        if ((match_std .or. match_twiss) .and. (ele%value(beta_a1$) <= 0 .or. ele%value(beta_b1$) <= 0)) then
+          call out_io (s_fatal$, r_name, &
+                        'ELEMENT: ' // ele_full_name(ele, '@N (&#)'), &
+                        'WHICH IS A MATCH ELEMENT HAS A BETA_A1 OR BETA_B1 THAT IS NOT POSITIVE.')
+          err_flag = .true.
+        endif
 
-      if (match_std .and. (ele%value(beta_a0$) <= 0 .or. ele%value(beta_b0$) <= 0)) then
-        call out_io (s_fatal$, r_name, &
-                      'ELEMENT: ' // ele_full_name(ele, '@N (&#)'), &
-                      'WHICH IS A MATCH ELEMENT HAS A BETA_A0 OR BETA_B0 THAT IS NOT POSITIVE.')
-        err_flag = .true.
+        if (match_std .and. (ele%value(beta_a0$) <= 0 .or. ele%value(beta_b0$) <= 0)) then
+          call out_io (s_fatal$, r_name, &
+                        'ELEMENT: ' // ele_full_name(ele, '@N (&#)'), &
+                        'WHICH IS A MATCH ELEMENT HAS A BETA_A0 OR BETA_B0 THAT IS NOT POSITIVE.')
+          err_flag = .true.
+        endif
       endif
 
       if ((match_twiss .or. match_orbit) .and. ele%value(delta_time$) /= 0) then

bmad/doc/cover-page.tex

@@ -3,7 +3,7 @@
 
 \begin{flushright}
 \large
-  Revision: July 30, 2024 \\
+  Revision: August 13, 2024 \\
 \end{flushright}
 
 \pdfbookmark[0]{Preamble}{Preamble} 

bmad/doc/elements.tex

@@ -3492,7 +3492,9 @@ \section{Match}
 (and only if) the actual Twiss and coupling parameters at the entrance of the \vn{match} element
 are equal to the element entrance Twiss and coupling parameters, then the computed Twiss and
 coupling parameters at the exit end of the \vn{match} element will be the element end Twiss and coupling
-parameters. Additionally, the phase advances (in radians) will be \vn{dphi_a} and \vn{dphi_b}.
+parameters. Additionally, the phase advances (in radians) will be \vn{dphi_a} and \vn{dphi_b}. Exception:
+If \vn{beta_a0}, \vn{beta_b0}, \vn{beta_a1}, and \vn{beta_b1} are all zero, the matrix will be set to the
+identity.
 %
 \item[identity] \Newline
 With \vn{matrix} set to \vn{identity} the transfer matrix will be set to the unit matrix independent

bmad/low_level/match_ele_to_mat6.f90

@@ -66,7 +66,8 @@ subroutine match_ele_to_mat6 (ele, start_orb, mat6, vec0, err_flag, include_delt
   v(pz0$) = start_orb%vec(6)
 endif
 
-if (m_val == identity$ .or. ((t_ele%a%beta == 0 .or. t_ele%b%beta == 0) .and. (do_twiss .or. do_trombone))) then
+if (m_val == identity$ .or. ((t_ele%a%beta == 0 .or. t_ele%b%beta == 0) .and. (do_twiss .or. do_trombone)) .or. &
+    (m_val == standard$ .and. v(beta_a0$) == 0 .and. v(beta_b0$) == 0 .and. v(beta_a1$) == 0 .and. v(beta_b1$) == 0)) then
   v(mode_flip0$) = int_logic(t_ele%mode_flip)
   v(mode_flip1$) = int_logic(t_ele%mode_flip)
   call mat_make_unit(mat6)

bmad/parsing/bmad_parser_mod.f90

@@ -6012,7 +6012,12 @@ subroutine compute_super_lord_s (ref_ele, super_ele, pele, ix_insert)
 case default
   s_ref_begin = ref_ele%s_start
   s_ref_end = ref_ele%s
-  ix_ref = ref_ele%ix_ele
+  if (ref_ele%n_slave > 0) then
+    slave => pointer_to_slave(ref_ele, 1)
+    ix_ref = slave%ix_ele
+  else
+    ix_ref = ref_ele%ix_ele
+  endif
 end select
 
 ! Now compute the s position at the end of the element and put it in ele%s.

tao/version/tao_version_mod.f90

@@ -6,5 +6,5 @@
 !-
 
 module tao_version_mod
-character(*), parameter :: tao_version_date = "2024/08/09 00:00:04"
+character(*), parameter :: tao_version_date = "2024/08/10 16:53:27"
 end module